Worked example - building a proton. A proton is made of two up quarks and one down quark (uud), glued together by gluons. Let's check its electric charge. An up quark carries charge +2/3 (in units of the electron's charge) and a down quark carries -1/3. Adding them: (+2/3) + (+2/3) + (-1/3) = +3/3 = +1. So the proton has charge +1, exactly as measured. Now try a neutron: one up and two downs (udd): (+2/3) + (-1/3) + (-1/3) = 0. Charge zero - correct again! This simple fraction arithmetic is one of the strongest pieces of evidence that quarks are real.