The human body is a fascinating topic that can command our attention for hours, but there is no organ quite as fascinating and unbelievable as that of the human brain. Weighing, on average, a mere 3 pounds, the human brain acts as ‘command central’ for our entire bodies, determining how we act, react, how we feel, interpreting our senses and telling our muscles how to move and function. Containing approximately 100 billion neurons or nerve cells, it can process information as slowly as 0.5m/sec or as fast as 120m/sec, a feat that many of us take for granted!

As Brett Wingeier, Ph.D., an engineer, neuroscientists and the co-founder of Halo Neuroscience explains, “your brain is constantly working – to sense, process, think, move, and even dream.” By understanding how the neurons connect, which electrical impulses carry messages to various organs in the body and which area of the brain is associated with our various senses we can begin to wrap our head around its incredible function, but what about the topics of intelligence, problem-solving and critical thinking skills?

Experts say that you can actually strengthen your brain’s function in a similar way to an athlete strengthening their muscles working out. By reading, doing puzzles and challenging our ability to solve various problems we can keep our brain active and working in its optimal condition. The internet is packed with amazing examples of critical thinking and problem-solving riddles, each pushing us to think outside the box and expand our mental capacity.

Here are 22 incredible thought experiments to challenge your mind:

#1 – Recently featured in an episode of Dr. Who, the Bootstrap Paradox was named after a time travel story written by Robert A. Heinlein called ‘By His Bootstraps.’ This poses the idea that if an object or piece of information is sent back it in time it will become trapped in an infinite loop, never actually having a noticeable origin.

For example, if the Doctor in Dr. Who goes back in time and gives a young Beethoven a copy of his famous Sonatas, and he publishes them, who actually composed these works?

#2 – Roman poet and philosopher Titus Lucretius Carus posed the following question which continues to leave many scratching their heads today:

“If atoms in our brain always act predictably, how can we have free will?”

#3 – Popularized by John Maynard Keynes, a renowned economist, the ‘Paradox of Thrift’ introduces an interesting and complex problem that underlies our current economic system. It proposes that during difficult economic times everyone should save their money as it is the responsible thing to do, avoiding spending on things that we do not need. However, if each of us starts saving all of our money and cutting our unnecessary spending then that will cause the aggregate demand to drop, and as the impact of that shift takes investments will give lower return and income will also decline meaning that we will, ultimately, have fewer savings.

#4 – The following paradox was based on a true story. Known as the ‘Paradox of the Court,’ it tells the famous conundrum of Protagoras and his pupil Euathlus. Consider this one seriously and try applying it to a more modern setting!

“John taught law to Bill with the condition that Bill would pay the tuition fee after he had won his first case. Long after Bill finished his lessons, he did not get any case – either on purpose so that he never had to pay for the lessons or simply because no one hired him. John then sued Bill in small claims court for the tuition fee.

John: If I win the suit, then by verdict Bill will have to pay me; however, if Bill wins, he will also have to pay me because our agreement says he will pay me when he’s won his first case. As such, it would be wise for the court to simply make Bill pay me.

Bill: If I win the suit, then by verdict I needn’t pay John. If John wins the case, then I still won’t have to pay him because I haven’t won any cases. As such, it would be wise for the court to throw out his suit.

You’re the judge; what do you do?”

#5 – Classical philosopher and poet Epidmenides once wrote ‘all Cretans are liars.’ While this may seem like a simple phrase it was the birth of a new paradox, Epidmenides’ Paradox. You see, Epidmenides himself was a Cretan, therefore he is stating that he too is a liar. However, if he is a liar, then his statement that ‘all Cretans are liars’ must also be a lie meaning that all Cretans tell the truth. This would then state that Epidmenides must tell the truth… A circular pattern of thinking leaves one trying to determine the phrase ‘all Cretans are liars’ can be both true and false at the same time.


#6 – Zeno was a Greek philosopher in around 500 B.C.E. who proposed a number of interesting thought experiments and paradoxes, calling many areas of common thinking and beliefs into question. One of these areas was that of motion and its existence or lack thereof according to what he proposes as logical thinking. One of his famous ‘Paradoxes of Motion,’ the paradox of the arrow states this:

“For motion to occur, an object must move from one place in space/time to another. Using an arrow as a reference, if an observer were able to stop time, you would notice that the arrow was not moving in that instant. The arrow had to be either moving to where it is, or moving to where it isn’t, yet in any given instant the arrow is motionless. If in any instant no motion is occurring, then no motion can occur in any moment, and therefore motion cannot happen.”

#7 – Introduced by the Muslim philosopher Avicenna, the ‘Floating Man Problem’ questions our real existence in relation to our thoughts, memories, and experiences. This poses the question, what truly defines existence? It asks:

“Imagine a man is created in total sensory deprivation. He cannot see, hear, taste, smell, or touch. Can he know that he exists?”

While Avicenna himself assumed the answer would be yes, pointing to the existence of the human soul, this question has continued to circulate among philosophers and not all shared his conclusion.

#8 – In an effort to point out how different legal consequences can follow a homicide case depending on the twists that the situation may take, Don Harper Mills, the president of the American Academy of Forensic Sciences introduced the fictional murder case of Ronald Opus. Here is the original case in its entirety. If you were responsible for persecuting this case, what would your conclusion be?

“On March 23, 1994, a medical examiner viewed the body of Ronald Opus and concluded that he died from a gunshot wound of the head caused by a shotgun. Investigation to that point had revealed that the decedent had jumped from the top of a ten-story building with the intent to commit suicide. (He left a note indicating his despondency.) As he passed the 9th floor on the way down, his life was interrupted by a shotgun blast through a window, killing him instantly. Neither the shooter nor the decedent was aware that a safety net had been erected at the 8th-floor level to protect some window washers, and that the decedent would most likely not have been able to complete his intent to commit suicide because of this.

Ordinarily, a person who starts into motion the events with a suicide intent ultimately commits suicide even though the mechanism might not be what they intended. That he was shot on the way to certain death nine stories below probably would not change his mode of death from suicide to homicide, but the fact that his suicide intent would not have been achieved under any circumstance caused the medical examiner to feel that he had homicide on his hands.

Further investigation led to the discovery that the room on the 9th floor from whence the shotgun blast emanated was occupied by an elderly man and his wife. He was threatening her with the shotgun because of an interspousal spat and became so upset that he could not hold the shotgun straight. Therefore, when he pulled the trigger, he completely missed his wife, and the pellets went through the window, striking the decedent.

When one intends to kill subject A but kills subject B in the attempt, one is guilty of the murder of subject B. The old man was confronted with this conclusion, but both he and his wife were adamant in stating that neither knew that the shotgun was loaded. It was the longtime habit of the old man to threaten his wife with an unloaded shotgun. He had no intent to murder her; therefore, the killing of the decedent appeared then to be accident. That is, the gun had been accidentally loaded.

But further investigation turned up a witness that their son was seen loading the shotgun approximately six weeks prior to the fatal accident. That investigation showed that the mother (the old lady) had cut off her son’s financial support, and her son, knowing the propensity of his father to use the shotgun threateningly, loaded the gun with the expectation that the father would shoot his mother. The case now becomes one of murder on the part of the son for the death of Ronald Opus.

Now comes the exquisite twist. Further investigation revealed that the son, Ronald Opus himself, had become increasingly despondent over the failure of his attempt to get his mother murdered. This led him to jump off the ten-story building on March 23, only to be killed by a shotgun blast through a 9th story window.

The medical examiner closed the case as a suicide.”

#9 – Here’s one that will really get you thinking. While the exact origin of this picture is unknown, it has circulated a number of social media platforms including Facebook, Twitter, and Google+. This one may have you stumped for a bit! How would you answer this riddle?


#10 – In 1982 philosopher Frank Jackson came up with a thought experiment known as ‘What Did Mary Know?’ this as used to argue the association between consciousness and non-physical knowledge. Is it possible that there are some ‘facts’ in life that escape a real physical explanation?

“Imagine a girl called Mary. She is a brilliant neuroscientist and a world expert on color vision. But because she grew up entirely in a black and white room, she has never actually seen any colors. Many black and white books and TV programmes have taught her all there is to know about color vision. Mary knows facts like the structure of our eyes and the exact wavelengths of light that stimulate our retinas when we look at a light blue sky.

One day, Mary escapes her monochrome room, and she walks through the grey city streets. She sees a red apple for the first time.

What changes upon Mary’s encounter with the red apple? Has Mary learnt anything new about the color red upon seeing the color for the first time?”


#11 – The Barn-Pole Paradox is one of a number of similar special relativity problems that use various props to make the same point. Approaching a situation from multiple points of view, it poses an interesting question that many find difficult to answer. The problem goes like this:

“These are the props. You own a barn, 40 m long, with automatic doors at either end, that can be opened and closed simultaneously be a switch. You also have a pole, 80m long, which of course won’t fit in the barn.

Now someone takes the pole and tries to run (at nearly the speed of light) through the barn with the pole horizontal. Special Relativity (SR) says that a moving object is contracted in the direction of motion: this is called the Lorentz Contraction. So, if the pole is set in motion lengthwise, then it will contract in the reference frame of a stationary observer.

You are that observer, sitting on the barn roof. You see the pole coming towards you, and it has contracted to a bit less than 40m in your reference frame.

So, as the pole passes through the barn, there is an instant when it is completely within the barn. At that instant you close both doors simultaneously, with your switch. Of course, you open them again pretty quickly, but at least momentarily you had the contracted pole shut up in your barn. The runner emerges from the far door unscathed.

But consider the problem form the point of view of the runner. She will regard the pole as stationary, and the barn as approaching at high speed. In this reference frame, the pole is still 80m long, and the barn is less than 20m long. Surely the runner is in trouble if the doors close while she is inside. The pole is sure to get caught.

Well, does the pole get caught in the door or doesn’t it?”

#12 – Greek philosopher Socrates was once quoted saying, “I know one thing; that I know nothing.” While his intention was to highlight that the more we know, the more we don’t know, with the acquisition of more knowledge only leading us to more unanswered questions, the whole concept can leave you thinking for hours. Based on what you currently ‘know’ today, what do you not know? What will you not know tomorrow?

#13 – The ‘Ship of Theseus’ or ‘Theseus’s Paradox’ is a thought experiment most notably recorded by Plutarch in the last first century. The original paradox stated:

“The ship wherein Theseus and the youth of Athens returned from Crete had thirty oars, and was preserved by the Athenians down even to the time of Demetrius Phalereus, for they took away the old planks as they decayed, putting in new and stronger timber in their places, in so much that this ship became a standing example among the philosophers, for the logical question of things that grow; one side holding that the ship remained the same, and the other contending that it was not the same.

Modern takes on this paradox include a sock where the holes are replaced with patches, or ‘grandfather’s axe’ which, over time, has had both the handle and the head replaced. Is it still considered the original axe, or is it now a new object?

#14 – Irish scientist William Molyneux posed this question in 1688 and it still has many of us thinking today!

“If a man who is born blind learns to distinguish between a sphere and a cube by touching them, and one day regains his sight, would he be able to tell which of the objects placed in front of him is a sphere and a cube by only looking at them?” What do you think?


#15 – Olbers’ Paradox was named after German astronomer Heinrich Wilhelm Olbers. Also known as the ‘Dark Night Sky Paradox’ it argues that the darkness of the night sky conflicts with the stated fact that there is an infinite number of stars in the universe. By this theory of the infinite stars, any line of sight from Earth must, ultimately, end at the surface of a star, which would, by definition, be bright. So why is the sky that we observe dark?


#16 – A story first used by American philosopher Edmund L. Gettier in his discussions about the concept of ‘justified true belief,’ this is sure to leave you wondering!

“A near-sighted farmer anxiously asked the milkman whether he’d seen a cow on his rounds because his cow was missing. The milkman pointed further down the road and told the farmer, “Don’t worry, she’s over there in that field.” The farmer squinted and saw a black and white shape near some trees in the middle of the field and thanked the milkman in relief. Meanwhile, as the milkman passed the field he took another look and notice that the cow was actually hidden by high grass; what he’d pointed out to the farmer were black and white sheets hanging in a tree. Even though the cow was in the field, was the milkman correct when he’d said he knew it was there?”

#17 – While discussing the topic of personal identity, British philosopher Derek Parfit introduced what has now come to be known as the ‘Teletransporter Thought Experiment.’ It asked this – if you were to use a teleporter in order to travel to Mars, and this specific device mapped out every particle within your body, creating an exact replica of yourself on Mars while destroying the original here on Earth, maintaining your memories along with the memories, and others have no doubt in meeting you that this is you… From a ‘first-person perspective’ having used the teleporter, would you continue to exist or are you actually dead? An interesting thought with the growing interest in new forms of space travel in our future!

#18 – Consider this question first asked by Archytas in 5th Century BC: “If I arrived at the outermost edge of the heaven (universe), could I extend my hand or staff into what is outside or not?” This raises a seriously interesting question… What exactly is the Universe expanding into? Is it a void or nothingness? If so, wouldn’t that void also be part of our Universe?

#19 – Have you ever stopped to consider the existence of ‘intention’ and what it truly means? This is what as called into question by moral and political philosopher Gregory S. Kavka in ‘The Toxin Puzzle,’ first introduced in 1983. It states that:

An eccentric billionaire places before you a vial of toxin that, if you drink it, will make you painfully ill for a day, but will not threaten your life or have any lasting effects. The billionaire will pay you one million dollars tomorrow morning if, at midnight tonight, you intend to drink the toxin tomorrow afternoon. He emphasizes that you need not drink the toxin to receive the money; in fact, the money will already be in your bank account hours before the time for drinking it arrives, if you succeed. All you have to do is. . . intend at midnight tonight to drink the stuff tomorrow afternoon. You are perfectly free to change your mind after receiving the money and not drink the toxin.”

The question remains; however, can you actually intend to drink the toxin if you know that you intend to change your mind at a later time? Or does intending to change your mind negate your claim to intend to drink it?

#20 – The crocodile dilemma, also known as crocodile sophism, is a paradox that dates back to ancient Greece. It is believed to be logically impossible, however, it is sure to get you thinking! The paradox goes like this:

“A crocodile has captured a little boy. Being the reasonable crocodile that he is, the croc promises the little boy’s father that he will release the kid only if the father can predict what the crocodile will do next. The father of the boy says, “You will not give my son back.” Now the croc is in a bind. If the feather was correct in his statement, the croc keeps the kid. But if the croc keeps the kid, the croc is not keeping his promise to return the boy. But once the croc returns the boy, the father’s prediction is no longer correct. What should the crocodile do?”


#21 – Referred to as ‘The Trolley Problem,’ this ethical dilemma has stumped people for generations. Sometimes told with a trolley, other times told with a train, the question is raised, will you personally take a life to save the lives of many?

“There is a runaway trolley barreling down the railway tracks. Ahead, on the tracks, there are five people tied up and unable to move. The trolley is headed straight for them. You are standing some distance off in the train yard, next to a lever. If you pull this lever, the trolley will switch to a different set of tracks. However, you notice that there is one person on the side track. You have two options: (1) Do nothing, and the trolley kills the five people on the main track. (2) Pull the lever, diverting the trolley onto the side track where it will kill one person. Which is the correct choice?”


#22 – It feels wrong to end this list of mind-provoking paradoxes without the best-known paradox in existence… The Chicken and Egg Paradox. This age-old question sheds light on two very important areas of consideration: the origin of life and the universe, and the case for the circular cause and consequence logical fallacy which can be defined as, “when the consequence of the phenomenon is said to be an unavoidable cause of the phenomenon when the truth may be otherwise.”

So, the question stands – What came first, the chicken or the egg?

Featured image via Medium

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