"Under some situations it is possible for 2 electrons to couple to form a pair. This is what happens in a superconductor. Here 2 electrons attract each other weakly despite the fact that since they carry negative charges, they ought to repel each other.
This weak attraction is mediated in some superconductors by the phonons which are the quantized excitations of atoms. Since a phonon is electrically neutral, it can provide a partial shielding of the negative charge carried by an electron.
Therefore 2 electrons with opposite spins can form a weakly bound pair. This pair, known as the Cooper pair, as a whole has a zero spin because the 2 spins of electrons completely cancel each other.
So this pair is effectively a spin zero particle or a boson. It is not an elementary boson but a composite one. All particles with integer spins are known as bosons. This is how 2 fermions can give rise to a (composite) boson.
Unlike a fermion, a boson does not obey Pauli's exclusion principle. It means there is no limit on how many bosons may occupy a single quantum state. "
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