The neutron is a bit of a headache for physics. A neutron is an electrically neutral particle that helps glue protons together in the nucleus of atoms. Inside the atom, it is happily stable.
But a neutron alone is an unhappy beast. After about 10 minutes, it will emit an electron and an antineutrino and turn into a proton. The decay is all good, but there’s a problem with the “about 10 minutes” part of things. In one set of experiments, we have determined that the half-life of a lonely neutron is 879.6s. But, in another set of experiments, we’ve found that the neutron has a half-life of 888s (these numbers might be slightly out of date now). The chance of these two being different by accident is now about one in 100,000.