To illustrate the concept of exchange interaction, any two electrons, for example, in the universe are considered indistinguishable particles, and so according to quantum mechanics in 3 dimensions, every particle must behave as a boson or a fermion. In the former case, two (or more) particles can occupy the same quantum state and this results in an exchange interaction between them in the form of attraction; in the latter case, the particles can not occupy the same state according to the Pauli exclusion principle. From quantum field theory, the spin–statistics theorem demands that all particles with half-integer spin behave as fermions and all particles with integer spin behave as bosons. Thus, it so happens that all electrons are fermions, since they have spin 1/2.