Hyperbolic planes in easy to understand form

This site has a great article on crochet and mathematics, in words that all of us should be able to follow. The first article is about "Knitting for Peace day", after reading that, skip down below the crocheted leprechaun to "It's a crochet kind of Day" posted on March 17. The author's explanation of how to crochet flat circles, and curving bowls leads naturally into crocheting hyperbolic planes. She also nicely explains why you can crochet hyperbolic planes, but it's almost impossible to knit them.