Pionium

Okay, imagine a pion – it's a very small particle. Now imagine another identical pion coming close to the first pion. When these two pions get close, they can sometimes stick together and make a new particle called pionium!

Pionium is a bit like a special puzzle piece – the two pions fit together perfectly to make a new shape. This new shape doesn't last very long, though – the two pions will eventually separate again, and when they do, they will give off energy in the form of light.

Scientists like to study pionium because it helps them understand how particles stick together and interact with each other. By studying pionium, scientists can learn more about how the basic building blocks of everything in the world fit together.