This is a fun geometric problem with a practical solution:
How can one measure the diagonal of a brick without any formula, using three bricks and a ruler?
Can you solve it?
There is a remarkably easy way to find the diagonal. What you do is place two bricks in a row, and then stack one brick on top of either one. So you have one brick, and then next to it two bricks, creating an empty space where a fourth brick could be placed.
Now measure the length of the diagonal on the empty space using a ruler. No Pythagorean Theorem required!