Here's a classic dissection puzzle that always seemed impossibly difficult to me. Puzzle books always stated the puzzle and gave the answer but never explained how to work out the answer. You may find that my explanation will help you understand how to approach a puzzle like this.

An L-shaped plank of wood is really the size of two squares sharing a common edge. The object is to make two straight cuts and reassemble the resulting pieces to form one large square table top. You won't need to turn any of the pieces upside down, just shuffle the pieces in a sort of mini-jigsaw puzzle.

You don't need to know the dimensions of the two squares. It turns out that the solution is the same for any combination of dimensions.