Mesons are tiny sub-atomic particles. Sub-atomic means they are smaller than atoms. They form out of quarks in high-energy situations. Mesons are about 2/3 the size of protons or neutrons. A meson is a kind of boson. All mesons are unstable: they form and break up again in less than a few hundredths of a microsecond. Mesons with an electrical charge decay into electrons and neutrinos; mesons without any electrical charge can decay into photons.