Helium is the second most abundant element in the universe after hydrogen, but it is very rare on Earth. It is a colorless, odorless, and non-flammable gas that is lighter than air. This makes it ideal for filling balloons, as it causes them to float.

Helium is also used in a variety of other applications, including:

- In airships and balloons: Helium is used to inflate airships and balloons because it is lighter than air, and is resistant to fire.

- In breathing mixtures: Helium is used in breathing mixtures for deep-sea diving because it is less soluble in blood than nitrogen, which can cause decompression sickness (the bends) if it is breathed at high pressures.

- In cryogenics: Helium is used to cool materials to very low temperatures, including those used in superconducting magnets and in the Large Hadron Collider at CERN.

- In lasers: Helium is used in some types of lasers, such as helium-neon lasers.

- In party balloons: Helium is used to fill party balloons because it causes them to float.