We're accustomed to the idea that what starts as potential energy can convert to kinetic energy, or vice versa, but as Einstein pointed out there are actually three terms in the total energy expression... what is the third?

Ε = ∑i mic2 + E,

where Ε is the total energy of the system, and is equal to Mc2 if the system is at rest, mi is the mass of system constituent i, and E is the usual total energy, the sum of all kinetic and potential energies of the particles that make up the system. Since for a bound system E < 0, we see that  M < ∑ mi!

Standard energy unit in atomic and subatomic physics: the eV, which is 1.6 × 10-19 Joules.