Aerogel.

Known mostly for its use in gently catching particles ejected from Comet Wild 2, aerogel is a sort of foamy glass. Both are silicon-based solids yet aerogel is only 0.02 percent solid by volume – the rest is empty space! Weighing just 3 times as much as air, aerogel has earned itself the nickname “frozen smoke” and among its many claims to fame are 15 entries in the Guinness Book of World Records.