Experiments show that protons and neutrons consist of smaller parts
which called quarks.

A proton consist of 1 down quark and 2 up quarks.

  A neutron consists of 2 down quarks and 1 up quark.

  There are 6 types of quarks 

Up                Charm              Top
Down        Strange            Bottom

Quarks can only be "seen" during nuclear reactions.