If you've seen Kjartan (the author) talking about the Murderous Maths books,
he probably demonstrated some of his different FLEXAGONS.
A flexagon looks like a piece of folded card which you can turn
inside out. The good bit is that you're never quite sure what you'll see
If you want to know more, watch the video!
You can also find on-screen step by step instructions on
The Fabulous Flexagons page.
You can make the square
BABYFLEX and MONSTERFLEX
flexagons in The Magic of Maths