A new substance called aerogel, invented in the 1930s but recently refined by NASA, has been certified as the lightest solid in the world yes, it's in the Guinness Book of World Records. Weighing in at a mere .00011 lbs. per cu. in. (thin air weighs about .00004 lbs. per cu. in.), aerogel resembles smoke that has been frozen into place it's cloudy, translucent and virtually weightless. It's also surprisingly tough. Chemically similar to glass, aerogel is used on the space shuttle to trap tiny spaceborne particles traveling at high speed so they can be brought back to Earth for analysis.
To Learn More: www.jpl.nasa.gov/technology/features/aerogel.html