Archimedes tells us the lifting power of a balloon (how much mass it can lift) is equal to the difference between the mass of the balloon and the mass of the air it displaces. That is, if the balloon occupies the same volume as of air, but the balloon only weighs , then the balloon can lift . Airships have sometimes been filled with hydrogen (), but hydrogen is very flammable, and after the Hindenburg caught fire and crashed in with serious loss of life airships have nearly always been filled with helium () instead.