Pairing of electrons can dramatically change the properties of materials. When electrons flow in a normal conductor, they feel friction, which causes resistance and heating. At very low temperature, however, electrons can form pairs and flow without friction, forming a superconductor. Superconductivity is very similar to superfluidity, but involves charged particles. Pairing of unequal partners, in the case above of electrons with spin up and spin down, is the essence of the transition to a superfluid state. At high temperature, the pairs break up and superfluidity is lost.