Helium


 * Helium is so light that Earth's gravity is not strong enough to hold onto it. When helium atoms are released into the atmosphere, they rise until they escape into space.
 * At temperatures close to absolute zero, helium consenses to a liquid with amazing properties - the properties of a superfluid, flowing with zero friction up and over the walls of containers.
 * Helium only exists in Earth's atmosphere because it is constantly resupplied from two sources - decay of radioactive elements on Earth, and cosmic rays, about 9% of which are high energy helium.