Artist's impression of a nuclear particle. This shows the particle (either a proton or a neutron) as being composed of three quarks. In a proton, there are two 'up' and one 'down' quark, in a neutron two 'down' and one 'up'. The quarks are held together by the strong nuclear force which is carried by the exchange of gluons. The colours in this image reflect the charges of the strong force: red, green and blue. Each proton or neutron has one quark of each colour" (charge). The quarks are surrounded by a quantum cloud of virtual particles."