According to theory, fermions have half-integer spins (an odd number of halves),
unlike bosons, which have integer spins.
Leptons such as electrons are fermions;
quarks are fermions.
The rules for adding odd numbers are analogous;
two odds make an even; an odd number of odds make an odd.
Composites of odd numbers of leptons and quarks are fermions.
Protons and neutrons are composed of three quarks, so they are fermions.
A helium-3 atom, with two protons, one neutron, and two electrons, is a fermion.
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