Without invoking the supernatural or science fiction, how is it possible for a person who was born in 1990 to have turned 20 years old in the year 2000? If you can’t provide an immediate answer, don’t feel bad. Some version of this problem has stumped a great many people — including Marilyn vos Savant, who reportedly has the highest IQ ever recorded (for whatever that’s worth).

We’ll get to the solution later. But first, a hint: it’s really quite simple, yet most people overlook it because their assumptions get in the way.

Here’s a similar scenario, one you may have heard before. A father and son are traveling in their car, and have an accident in which the father is killed. The son is brought into an emergency room, and the surgeon there looks at him and says, “I can’t operate on this boy — he’s my son!” How can this be? (If you don’t already know this one, you might want to stop reading and ponder it before continuing.)

Chances are, this riddle was more of a head scratcher decades ago when nearly all surgeons were men. But now it’s (at least somewhat) more common to have female surgeons too — so it might be easier to guess that the doctor is the boy’s mother.

Actually, even that may be an unwarranted assumption, which becomes more clear in cultural context. The doctor could be either male or female, and the parents could be either gay or hetero; furthermore, one or both parents could be adoptive rather than biological.

Riddles and puzzles often prod us to question our assumptions, which is precisely what makes them so challenging and rewarding. Humor also plays upon assumptions.

Consider a little joke, probably my favorite “clean” joke ever. One married couple is visiting another, and one husband is telling the other about a restaurant where he and his wife recently ate. But he has a problematic memory, and can’t recall the name of it, which is just on the tip of his tongue. “Give me the name of a flower”, he says to the other husband, who replies, “Marigold”. “No, that’s not it”, says the first. “Give me another.” “Um… daffodil?” “No, no that’s not it. Try again.” “Well… how about rose?” “Yeah, that’s it. Hey Rose, what was the name of that restaurant?”

Our assumption is that the name of a flower will be relevant to the name of the restaurant, not to the name of the wife. And this expectation is reinforced with each iteration — much in the same way that repetition is used to reinforce propaganda. That’s why so many jokes follow the “rule of three” formula: the first two iterations set up an expectation, and then the punchline defies that expectation to humorous effect.

But at their best, the challenging of assumption, even in jokes or riddles, is more than just fun; it helps, in at least some small measure, to prepare us for real-life situations in which assumptions suffocate reason.

All too often, assumptions congeal into “conventional wisdom”, which gets mistaken for “common sense”. But while conventional wisdom is certainly conventional enough, it’s often very far from wise. Although many such assumptions have been discarded, many still persist in our own era. And new ones are always arising to take their position in the canon of public misinformation.

One common example is the persistent belief that guns make us safer. After all, if you have a gun, then surely you’ll have a better chance of defending yourself against the bad guys. It’s only logical, right? But reality isn’t nearly so simple or predictable; the truth is that while guns certainly make some people safer in some circumstances, their overall effect is to increase the level of danger, not the level of safety. Statistically, being armed may make it more likely that you will become a target of crime yourself — and even far more likely that you will get shot.

All of this may strike you as counterintuitive. But quite often, facts are counterintuitive. It sounds counterintuitive to suggest that when you’re driving on a slick road and start to skid, you should turn your wheels in the direction of the skid. But that’s exactly what produces the best results.

Other common assumptions are that giving people money with no strings attached will foster indolence, indigence, and dependence on “handouts”; that banning abortion will help curb it; and that transgender athletes have an unfair advantage. But when tested in the real world, those assumptions crumble. Experiments with Universal Basic Income (UBI), in which destitute individuals were issued a small cash stipend, have been consistently successful in reducing poverty, unemployment and homelessness. Statistics indicate that those countries with strict abortion laws actually have higher rates of abortion than those with more liberal laws. And trans athletes have to deal with many disadvantages, such as reduced muscle mass and aerobic capacity.

Another assumption that has become all too common is the belief that “illegal” immigrants are “stealing our jobs”. Actually, literally everything the American public believes about undocumented immigrants is false, but this one in particular is an apt illustration of faulty assumption.

At first blush, it seems to be built on a solid chain of logic, as follows: (1) refugees come into the U.S. in desperate circumstances; (2) therefore, they need to work; (3) U.S. citizens and legal residents also need to work; (4) therefore, they must be competing for the same jobs; (5) therefore, when “illegals” take jobs, they are taking them away from “real Americans”.

But while the first three premises are sound, the reasoning is derailed at step 4. Undocumented immigrants are generally not competing for the same jobs as legal residents; instead, they often perform tasks that no American would touch, such as gutting chickens. Furthermore, the whole canard ignores the other side of the equation: the jobs that “illegals” create. Specifically, they create 1.2 jobs for every job they fill. So that would be a net gain of 20 percent even if it were indeed true that all the jobs they “take” are jobs that otherwise would go to citizens.

In his (highly recommended) book Collective Illusions, Todd Rose relates how in one particular conservative community, the mores and morals essentially were dictated by one person, a bluenose matron who, because she was so outspoken, was presumed to represent the convictions of the community as a whole. Thus, because she expressed her loathing of card games (even of the most tame non-gambling variety), other people assumed that their neighbors also disapproved, and accordingly abstained in order to conform to the false norm. It wasn’t until one brave soul spoke up and said that actually he, um, enjoyed playing cards that many other people acknowledged they did as well — and in many cases had been closeted card players all along. But if brave individuals do not speak up and decry the Emperor’s clothes, a society will remain immersed in its illusions indefinitely.

Now let’s get back to that riddle we posed at the beginning. When we hear that someone was born in 1990, we assume that 1990 designates a year. And that would even be a valid assumption in the vast majority of cases. But in this scenario, something clearly does not add up. Since it’s specified that 2000 is a year, and 20 refers to the person’s age in years, the only logical conclusion is that 1990 must refer to something else. And that something else is almost certainly a location. Further, given the use of the preposition “in”, that location is almost certainly an apartment or room number.

It’s quite excusable if you did not make this deduction; you’re in very good company. But most other assumptions are deflated much more readily. These days, we have an infinitude of facts at our fingertips, and setting the record straight often requires only a few minutes. And yet people often cling to their assumptions, true or false, as tenaciously as they would cling to lifesavers at sea. Not because of any difficulty in discrediting those beliefs, but because they have a fear of being proven wrong; and because they prefer the comfort of the familiar to the challenge of an altered mindset. (Indeed, people often identify so strongly with their assumptions that when those assumptions are questioned, in even the most benign and constructive manner, they respond with the most strident hostility. ) But that’s a harmful and dangerous habit.

If we don’t detect and overcome false assumptions, they can cause harm and/ or prevent benefits, for ourselves and others. Assumptions led to the “conventional wisdom” that the earth is flat, and is the center of the universe; that males are superior to females and Caucasians superior to all other races; that royalty is a pinnacle of virtue; and that anything you can’t see can’t hurt you. You probably assume that churches and scout troops are safe and nurturing environments for your children, and you’re probably right. But they are also common havens for sexual predators who can scar kids for life; and it would be foolish to ignore that fact.

For many centuries, it was “conventional wisdom” in the Christian world that women had one rib more than men — a belief stemming from the biblical anecdote of Eve being created from Adam’s rib. (In fact, I remember people still repeating this belief when I was a child — and perhaps they still do.) This stemmed from the assumptions that (a) everything in the Bible is literally true, and (b) Adam’s rib shortage would be passed on to his descendants even though its origin was surgical rather than genetic. By some accounts, it was not until the Sixteenth Century or so that people started to realize that they could examine the evidence that was literally right in front of them: they could just count ribs.

We always have a choice between following our assumptions blindly testing testing them against real-world evidence. If there’s one piece of advice that I could offer anyone, one suggestion about the key to acquiring (real) wisdom, it would be just two words: count ribs.