Helium

Helium is a noble gas: a colorless, odorless gas that hardly reacts with other elements. It is abundant in stars but scarce in Earth's atmosphere. Most helium on Earth forms as radioactive elements such as uranium and thorium decay in rocks; some collects in underground natural gas deposits. Industry recovers it from these gases by cooling the mixture until other gases turn to liquid, leaving helium to be purified. Helium makes balloons float because it is less dense than air. Its boiling point is the lowest of any element, so liquid helium can keep the powerful magnets in many MRI scanners cold enough to operate as superconductors, carrying electric current without resistance.