The old The String Around The World puzzle.
Try it BEFORE you google.
I have had much fun with this one since high school.
Imagine I had a very long piece of string: long enough to wrap it around the equator of the Earth. And I’ll do just that. Yikes — that’s about 40,000 kilometers (or 25,000 miles) of string! I will make sure its pulled completely tight and connect both ends to each other: the result is that it lies flat onto the surface.
Now let me extend the string with just one measly meter. Compared to its total length that’s not a lot, is it? Once again I pull it tight, but now, with the added meter, it has come off the ground just a tiny bit. Assume that this extra distance (between the string and the Earth’s surface) is equally divided and thus the same all around the globe. How much would you guess this distance is? Surely this must be in the order of nanometers or whatnot, right?
Another fun one is crazy 9s.
Multiply any number by 9. The answer is always 9 or some number which can be eventually reduced to 9 by adding over and over.
that’s a good one! It’s pretty obvious once you figure it out. I approached it from the way of, what extra length would I need to raise the string 1 m off the ground.
I once saw this same kind of question in a physics book talking about the coefficient of expansion of steel. If you circled the earth with a band of steel and fit it tightly to the earth, then lined up people all the way around the earth to breathe warm air on it, it would raise the ring something like 20 or 30 feet off the surface.