Hypernuclear physics differs from that of normal nuclei because a hyperon is distinguishable from the four nucleon spin and isospin. That is, a single hyperon is not restricted by the Pauli exclusion principle, and can sink to the lowest energy level. As such, hypernuclei are often smaller and more tightly bound than normal nuclei; for example, the lithium hypernucleus Λ Li is 19% smaller than the normal nucleus Li. However, the hyperons can decay via the weak force ; the mean lifetime of a free Λ is 263 ± 2 ps, and that of a Λ hypernucleus is usually slightly shorter.