ELI5: Explain Like I'm 5

Clairaut's theorem

Imagine you have a big piece of Play-Doh that you can mold and shape into different things. One day, you decide to make a mountain out of the Play-Doh. But why does the mountain have the shape it has?

Well, a scientist named Clairaut figured out that the shape of the mountain depends on some things we call "variables" - like how high you want the mountain to be or how steep you want its slopes to be. And here's the cool thing: no matter how you change these variables, there's a rule that always stays true.

This rule is called Clairaut's Theorem. It says that if you take a point on the surface of the mountain and you measure the curvature of the mountain at that point, you'll always get the same answer.

What does that mean, exactly? Think of the mountain as an apple. If you look at the apple from the top, you can see a round shape. But if you look at the apple from the side, you can see that it's curved too - it's not just a flat circle. The same thing happens with the mountain. No matter where you stand on it, the curve of the mountain's surface will be the same.

That's basically what Clairaut's Theorem says. It's like a math rule that helps us understand how things like mountains are shaped. Pretty cool, huh?