David Hilbert (1862-1943) was a famous German mathematician who developed theories in number and geometry. He set twenty-three problems, known as Hilbert's Problems , for mathematicians to solve. He believed that solving the problems would lead to new discovers and ideas in mathematics. Some people believe that Hilbert worked out all the field equations for 'general relativity' before Albert Einstein, but he didn't claim credit for this now very famous work.

Hilbert proved that any polygon could be cut up into a certain number of pieces that could be put back together to form any other polygon of equal area. For example, this means that you can take a square, and work out a way to cut it up so the pieces can be put together to make a hexagon. Or it could be cut up a different way to make a triangle, or a rhombus, and so on.