Space is filled with radiation. The particles of radiation from space are called cosmic rays. Where do cosmic rays come from? Many are the result of distant violent events, like storms on the Sun or even exploding stars. Cosmic rays are constantly crashing into the top of the atmosphere, producing new particles. Other cosmic rays make it all the way to the ground and are stopped by the rock beneath our feet. Some particles, called neutrinos, don't even stop there. They can pass right through Earth without leaving a trace!

A cloud chamber is a device that makes cosmic rays visible. It was invented by the physicist E.T.R. Wilson around 1900. Wilson was awarded the 1927 Nobel Prize in Physics for this invention.

A cloud chamber contains a sealed volume of air that is saturated with cold alcohol vapor. When an electrically charged particle (an electron, a proton, or an atomic nucleus) passes through the cloud chamber, the alcohol vapor condenses back into liquid droplets. The path of the charged particle is made visible as a trail of tiny droplets of liquid alcohol.

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