Helium is the second lightest element in the universe and the second most common, found in huge quantities in stars like our sun. On Earth, helium is a colorless, odorless gas best known for making balloons float and voices sound squeaky.

Helium floats because it's much lighter than the air around us. Regular air is mostly nitrogen and oxygen, which are heavier than helium. When you fill a balloon with helium, it rises for the same reason a beach ball pops up when you push it underwater: the heavier substance (air or water) pushes the lighter substance (helium or the ball) upward.

The squeaky voice effect happens because helium makes sound waves travel faster than they do through regular air. Your vocal cords vibrate the same way, but the sound comes out at a higher pitch, like a recording played at faster speed.

Scientists value helium for serious purposes too. It stays liquid at extremely cold temperatures, making it essential for cooling powerful magnets in MRI machines that help doctors see inside the human body. Welders use helium to protect hot metal from reacting with oxygen in the air. Deep-sea divers breathe special mixtures containing helium because it helps reduce the dangerous condition called “the bends” that regular air can cause under high pressure.

Unlike most gases, helium never becomes a solid at normal pressure, no matter how cold it gets.