I love the idea that some fancy mathematical ideas can best be explained with a crocheted piece of fabric - hyperbolic planes. Here's a video that explains the idea, an interview and here the paper.
Plus, it makes a great bathroom sponge/scrubby :-)
How to make it
If you want to crochet such a sponge, it's really easy to do: you only need some leftover cotton yarn and the appropriate crochet hook.
Do a magic ring with 6 single crochet stitches.
After that continue to crochet 2 sc into each single crochet (i.e. doubling the number of stitches every round) ... until it's big enough or you run out of yarn.