Juju has written an nice introductory piece about aerogel, a solid material that just a tiny bit denser than air. The pics are amazing. They look fake, but they come from the NASA web site. Link
Mark Frauenfelder is the founder of Boing Boing and the editor-in-chief of MAKE and Cool Tools. Twitter: @frauenfelder. Come and hear Mark speak at the ALA conference in Chicago on July 1.