A cloud chamber and the birth of helium atoms.
The tracks here show the natural radiation in the atmosphere around us. Thicker tracks are big heavy alpha particles, whispy ones are beta particles, but they all work in the same way.
The atmosphere in the chamber contains a lot of alcohol vapour. When charged particles fire through the atmosphere they cause little droplets to form, and these are the clouds you can see.
This is similar to how a track forms behind an aeroplane, where the exhaust can help the droplets of moisture form together into a cloud.
When you then add an Americium pole into the chamber, the alpha particles emitted (with their two protons and two neutrons) grab electrons and form Helium atoms.
Watch the full video with Dr Peter Wothers on YouTube.