Neutrons are one of the three particles that atoms are made from. In nature neutrons are not often found outside of atoms. In the core of a nuclear reactor however there are large numbers of neutrons produced from the fission of uranium. In each centimetre cube in a reactor there are 1,000,000,000 neutrons. Neutrons have no charge so they are not strongly attracted to atoms. Because atoms are mostly empty space, a neutron can pass through solid matter relatively easily. A probe that can pass through solid material can be used to give scientists information about that material. A simple example of that is an x-ray that passes through your body to leave an image on a film. Neutrons are more penetrating and more gentle than x-rays.