The Hindenburg was an airship that exploded and burned up as it came in to land in Lakehurst, New Jersey in 1937, after 67 hours flight. Even today, people ask, "Why did the Hindenburg burn?" and no clear-cut was given. However, one of the problems with hydrogen-filled balloons has been a diffusion of air into the blimp containing hydrogen to make a gas mixture. Since hydrogen is unstable in contact with oxygen, only a spark, for example by static electricity after a thunderstorm, is required to trigger the reaction. The leakage of balloon gas to the outside atmosphere was often considered to be the only important diffusion process. The diffusion of air into the balloon, however, is also a spontaneous process, since each of the gases behaves independently to proceed towards equilibrium. In an instructive experiment, designed for students not only the diffusion of helium from a helium-filled balloon, but also the diffusion of the air into the balloon was studied by measuring the change in the mole fraction in a rubber balloon inflated with helium using a precision mass spectrometer. The mole fraction of He in a balloon decreased to 0.346 (751.2 torr) from an initial mole fraction of 1.000 (770.3 torr). Meanwhile, the mole fraction of N2 and O2 in the balloon increased to 0.430 and 0.210 respectively, from an initial mole fraction of zero at an atmospheric pressure of 748.8 torr and temperature of 25 oC over a 24 h period, thus showing the diffusion of atmospheric gases into balloon. It is obvious that a rubber bag containing flammable gas is very dangerous. For this reason, helium, which is non-flammable, has today replaced hydrogen as gas used for filling balloons, although it is more expensive and has less lifting power.