Helium-4

Okay kiddo, helium-4 is a special gas that we sometimes use to make balloons go up, up, up into the sky. It's called helium-4 because it has four little particles called protons in its center, which we call a nucleus.

Now, these protons really like to stay close together and stick to each other like glue. That's why helium-4 is a gas, because the particles are really crowded and bouncing around a lot. And because the particles are bouncing around so much, helium-4 is also very light. That's why it makes balloons float - because it's lighter than air!

Helium-4 is also very important to scientists. They use it to cool down things that need to be super cold, like MRI machines in hospitals, and to study really tiny stuff like atoms and molecules. It's kind of like a superpower for scientists because they can make things really cold with it and study them better.

So kiddo, next time you see a helium balloon, remember that it's made of a special gas with four little particles sticking together like glue, and that it helps scientists do really cool things!