Quarks are fundamental particles which among other properties have an electrical charge and a name (e.g. 'up', 'down', 'strange'). The electrical charge of a single quark is always a fraction (e.g. 2/3). Quarks combine to make a hadron of integer charge, for example, the charges of two 'down' quarks and one 'up' quark combine like this -1/3 -1/3 +2/3 = 0 to make a "neutrally" charged "neutron". Quarks do not appear alone, they exist in groups and are held together by the strong nuclear force. The word to describe the different types of quarks is "flavours." According to the current standard model there are 6 flavours of quarks. There are two known ways quarks can combine, to give the two categories of hadrons: