Superfluid helium-4

Superfluid Helium-4 is a very special type of liquid. It is very cold, much colder than any liquid found in nature. When it is at this cold temperature (called a "critical temperature"), it starts to behave in special ways. Its molecules can start to move around freely, like they don't have any friction or resistance. It can also climb up and down walls and flow against gravity. So it's like a liquid that's got superpowers!