Mohamed from Maldives asks the following question: "Mr. Smith and his son are driving in a car. They get into an accident. Mr. Smith dies and his son rushed into the hospital and the surgeon said I cant operate on him because he is my SON. How is that?"

Hi Mohamed, this is an old, old riddle which is slowly disappearing from usage. Because of changes in society, people are becoming more and more likely to recognize the answer quickly. I'll explain what I mean in a minute, but first, the answer to your question:

The surgeon was his mother.

I first heard this riddle when I was a child in the early seventies. At the time, although I didn't know it, the percentage of doctors in the US who were female was less than 10%, and the percentage who were surgeons was even smaller.

In addition, every book I read that talked about doctors, always used "he" as the pronoun, and if there was a picture it was always something like the picture shown here.

Lab coat, stethoscope, maybe a black bag, but most importantly, guaranteed, it was a picture of a male doctor. True, there were female doctors, and female surgeons, when I was a child, but nobody ever pictured doctors as being female. Thus, the riddle was a bit tricky. Your brain automatically pictures a surgeon as male, because that's how you've been conditioned over time to picture surgeons!

In the US, where I live, things have changed a lot since the days when I was a kid. Now the percentage of surgeons who are female is significantly higher. Of course, it depends on the field of surgery; OB/GYN surgeons are almost 50% female. Other areas have a much smaller percentage of women in them. Pediatric surgery is just a little under 20% female.

So we don't tend to automatically assume that a surgeon is male, and the riddle seems less perplexing.

The percentage of women in surgical fields varies depending on where you live. In your part of the world, although there is a very high rate of females who attend medical school (I think I read that in one of the neighboring countries to yours, more than 50% of students enrolled in medical schools in 2015 were female), a very small percentages of them go into surgery. I'm not sure why that is; it would be an interesting sociological question to explore! Regardless, since the percentage of female surgeons is much smaller, the riddle didn't seem quite so obvious to you.

Someday, probably, this riddle will stop being told altogether, and children will learn the riddle by reading sites like this one, and they'll say, "What? People didn't know that women can be doctors?"

They say that if my hot chocolate is too hot to drink, I should stir it, because that helps to cool it down. But doesn't stirring increase the temperature by adding friction between the hot chocolate and the spoon?

Interesting question! You are correct that stirring will cause friction. Not just between the spoon and the hot chocolate, but also between the hot chocolate and the mug it's swirling around in. And, unless you are careful to avoid touching the spoon against the mug, there's also friction between the mug and the spoon.

And yes, if that's all that was happening, the temperature of your hot chocolate would rise (although probably not enough that you would notice it!).

But there's something else happening here. Heat is transferring from your hot chocolate to the cooler air above it.

And here's why stirring could make a difference: heat transfers more rapidly when there's a larger temperature difference. You know this, because your house loses heat more quickly in the winter than it does in the spring. If the temperature outside is 0ºC, you'll lose heat more rapidly than if the temperature was 15ºC.

Similarly, your hot chocolate is losing heat to the air above it. The air above your mug is warming up, while the hot chocolate is gradually cooling down.

The problem is, your hot chocolate is not losing heat uniformly. The surface is cooling down rapidly, while deep in the mug it's still very hot. You can test this for yourself. Scoop a spoonful of hot chocolate from the surface and drink it, then take a spoonful from deeper in the mug. You'll find that the deeper spoonful is hotter.  So the surface is cooling rapidly, while underneath it is not. As the surface of your hot chocolate cools down, and the air warms up, the temperature difference is decreasing. And if the temperature difference is decreasing, that means that the rate of heat loss is decreasing.

Meanwhile, the deeper-down hot chocolate is gradually losing heat to the cooler upper layer, but that's a slow process.

How to speed that process up? Stir your hot chocolate occasionally! That'll bring some of the hotter beverage to the surface. Now you'll have a higher temperature difference, and the rate of heat loss will increase.

Incidentally, while we're on the subject of heat loss, you can also help your hot chocolate cool down faster by moving it around on the table periodically. After all, every time you move it away from the air it has heated up, to cooler air, you've increased the temperature difference again. Will that significantly increase the rate of cooling? Probably not, but it'll make you feel like you're doing something useful while you wait, which is probably the real reason people stir in the first place!

Jakob saw a multiple choice question on a website, and was surprised by the answer. He was curious to know what Professor Puzzler thought.

Here's the question: You have an iron rod which is 1 light year in length. You push one end of the rod. How long does it take before the other end moves? The options are:

A: Immediately
B: 1 week
C: 1 year
D: at least 1,000 years.

The "instinctive" answer is to say A - the moment you move one end is the moment the other end moves. And, as you might guess, from the fact that we're being asked to explain it, A is not correct.

To help understand what's happening here, I'd like you to imagine that the rod is not made of iron. I want you to imagine instead that it's made of marshmallow. Yes, that's right - one single marshmallow that's 1 light year tall.

Now imagine what happens when you push one end. Can you visualize it? As you push, the marshmallow rod compresses (squishes) right by where you pushed it. Of course, it won't stay compressed, and since it can't "rebound" to its original position (your hand is in the way, and you're still pushing), the compressed section will decompress by pushing the compression down the length of the rod. In other words, you'll create a wave of compressed marshmallow that will travel the length of the rod.

"Ah, yes," you might say, "but that's marshmallow. A marshmallow is squishy. An iron rod isn't squishy. It's rigid."

Well, yes and no. It's kind of like "The Princess Bride," where Miracle Max concludes that "mostly dead is partly alive." Iron is "mostly rigid," and mostly rigid is partly squishy. It will compress. The motion of the rod is actually a wave of motion, just like with the marshmallow. The other end of your rod will move after you've pushed your end.

How much after? Well, that's a good question. And I don't know the answer. But I do know this: the speed of that wave is significantly slower than the speed of light. And how long does it take light to travel the length of the rod? Since the definition of a "light year" is "the distance light will travel in a year," we can conclude that light takes a year to travel the length of the rod. And therefore, the amount of time it takes for the other end of the rod to move is more than a year. If you look at your possible answers, D is the only one that is more than a year. Therefore, if any of the answers are correct, D must be the one!

It seems counter-intuitive; when you are dealing with much smaller "mostly rigid" objects, it appears to you as though the other end moves simultaneously with the end you pushed. The key is the word "appears." There is actually a tiny, tiny delay, which is so short you couldn't possibly measure it. But when you are dealing with such an astronomical distance (the length of your rod is 63 thousand times the distance from the earth to the sun!), that tiny, tiny delay is suddenly not tiny any more!

Incidentally, I hope you're planning to push very very hard, if you intend to move that rod of yours any significant distance. If we assume that the radius of the rod is 1 centimeter, its mass is about 23 quadrillion kilograms, which would make for a decent sized asteroid, if it wasn't so oddly shaped!

Dear Professor Puzzler,

Pick a three digit number. Reverse the digits. Subtract the smaller from the larger. Now add the digits of your result, and you get 18. Every time. How does this work?

Yvonne from Georgia

Hi Yvonne,

First of all, it doesn't work every time. Let's look at a counter-example first, and then we'll look at why it mostly works.

In order to make this not work, I picked a number in which the first and last digits were the same:

525. Reverse the digits and you get the same number. Subtract these and you get zero. Add the digits of zero, and you get...zero!

So, maybe you should rephrase your instructions to say that the digits have to be different, or that the first and last digits can't be the same.

Okay, so now let's look at the math behind what is happening.

Suppose the number I picked was abc, with a representing the hundreds place, b the tens place, and c the ones place.

Then the value of my number is 100a + 10b + c. And if I reverse the digits, the new number's value is 100c + 10b + a.

Now, we don't know which number is larger, so we'll assume the original number was bigger. If it was the other way around, we'd just have to reverse the sign of our result.

100a + 10b + c - (100c + 10b + a) =
100a + 10b + c - 100c - 10b - a =
99a - 99c =
99(a - c)

Isn't that interesting! Your result will always be a multiple of 99! Which means it's a multiple of 9 and a multiple of 11.

We have a divisibility rule for nine (you can find several divisibility rules here: Reference Unit on Divisibility Rules) which says that if a number is divisible by 9, its digits add to a multiple of 9. And 18 is certainly a multiple of 9!

From here, the easiest thing to do is to list out the multiples of 99 that are 3 digits or fewer, and you'll see that in each case, their digits add to either 0 or 18:

0 ⇒ 0
99 ⇒ 18
198 ⇒ 18
297 ⇒ 18
396 ⇒ 18
495 ⇒ 18
594 ⇒ 18
693 ⇒ 18
792 ⇒ 18
891 ⇒ 18
990 ⇒ 18

And there you have it! Except for the case of a repeated first and last digit, which gives you zero, every other possibility gives you 18!

Thanks for a fun question, Yvonne!

Professor Puzzler

Hi, there's a video going around in which a guy (Rick Lax) asks you to pick a state, and then he guesses what it is. I don't know how he does it. Kara

Well Kara, first of all, he doesn't do it. At no point in the video does he guess your state. He's a magician, and his goal is to do some misdirection, and if he fooled you into thinking he did guess your state, then he succeeded in his misdirection. Let me talk you though what he did, and then I'll talk you through how he did it (and by the way, don't expect that we're going to make a habit of doing "spoil the magician's trick" posts - this is a unique case).

First, he tells you to pick a state.

Next, he subtly steers you away from two states by using them as examples. If you picked Utah or New York as your state, when he started using those two as examples, you probably said, "Oh, I better pick something different." Ironically, if you'd stayed with your original pick, he would have gotten it wrong.

Next he tells you to take note of your state name, and the last letter in the name. Here's where he uses the examples of Utah and New York, with one being "H" and the other being "K." At this point he instructs you to "lock in" your guess by clicking "Like."

And this, right here, is the one thing about Rick Lax videos that I detest. This is painfully blatant like-baiting. All of his videos seem to have some degree of like-baiting, but this one is just obnoxious. Rick, if you ever read this post, please stop doing stuff like this. Your videos are fun, and people will share them without you doing like-baiting. 

After you've rebelled against being told to "like" the video, and sat there sullenly waiting for him to continue, eventually he puts a bunch of letters on the screen (as shown above) and asks you to find your letter. After he gives you a few seconds, he tells you that your letter is RED.

Wow! How did he do it? Well, unless you picked Wyoming (the G is blue) or Utah or New York, every other state name in the United States ends in A, D, E, I, N, O, S,T, or Y.

All of which are red. In other words, there are only three states you can pick which don't end with a red letter.

Why didn't he just make the H, K, and G red? Maybe because that would be too much red (as it is, he stuffed the whites and blues by including the letter H three times). Maybe because he figured that getting it right 47 out of 50 times is more impressive than being right all the time. After all, if he's right 100% of the time, people might try harder to figure out the trick, but if he's not 100% correct, people will think he's just an amazing guesser.

I don't normally "spoil" magicians' tricks on my blog, but the like-baiting means that this video will keep showing up on people's news feeds not because it's so good, but because they get "pushed" into liking it. Really, Rick. Seriously. The like-baiting and share-baiting makes you look pathetic, and you're really not - you're better than that! One of the best comments I saw on your video was: "You'd make a great used-car salesman." Really. That's not how you want to be seen!

So, if you like his videos, then "like" them. Don't "like" them because he told you that you have to!

Professor Puzzler

P.S. - He's got some videos that have some fun number theory in them, and I'd like to do posts on them, just because I love number theory, but I hate spoiling the fun!