This one ounce sample of aerogel is among the most impressive materials humans have created ... so far.
Aerogels are a group of materials in which the liquid portion of a gel has been replaced with a gas. Commonly called "frozen smoke", it is 99.8% empty space. It is safe to touch, hydrophobic (repels water), and an outstanding insulator.
Aerogel is a solid but on the nanoscale it has a mesh or sponge-like structure. The struts of this structure are nanoscale, as the pores are at around 20nm across, making it incredibly light.