The above has a relevance here because in this post I wish to try and explain in as simple terms as possible how subnuclear particles decay. Why do particles decay instead of holding on to their mass forever? Composite systemsSubnuclear particles making up matter are either truly elementary, or they are built as a combination of some elementary blocks. The picture below shows the elementary particles, both matter ones (quarks and leptons) and force carriers. The neutron decay is a case when the “phase space” for its only possible disintegration mode is very small.