Heating fullerenes at 650 degrees C under 3000 atmospheres of the noble gases helium, neon, argon, krypton, and xenon introduces these atoms into the fullerene cages in about one in 1000 molecules. A "window" mechanism in which one or more of the carbon-carbon bonds of the cage is broken has been proposed to explain the process. The amount of gas inside the fullerenes can be measured by heating to 1000 degrees C to expel the gases, which can then be measured by mass spectroscopy. Information obtained from the nuclear magnetic resonance spectra of helium-3-labeled fullerenes indicates that the magnetic field inside the cage is altered by aromatic ring current effects. Each higher fullerene isomer and each chemical derivative of a fullerene that has been studied so far has given a distinct helium nuclear magnetic resonance peak.