One of the strange consequences of quantum mechanics is the phenomenon of indistinguishability—that two quantum particles can be impossible to tell apart, even in principle. This happens, in part, because it is impossible to determine the precise position of quantum particles. So when two particles interact at the same location, there is no way of knowing which is which.

That gives rise to some exotic behavior, particularly at low temperatures when large numbers of particles can behave in the same way. The indistinguishability of photons makes lasers possible, the indistinguishability of helium-4 nuclei at low temperature leads to superfluidity and the indistinguishability of other nuclei like rubidium leads to Bose-Einstein condensates. Indistinguishability is rich in mysterious phenomena.

But some quantum particles are not indistinguishable in this way. Electrons, for example, are forbidden from sharing the same state by a law known as the Pauli exclusion principle. And that leads to a different kind of physics. The interactions between electrons, governed by this Pauli exclusion principle, is called chemistry and it is equally rich in exotic behavior.