Hydrogen: Orto and Para Hydrogen
Two atoms of hydrogen can combine to form a very stable hydrogen molecule. However, Heisenberg proved that, if we consider the nuclear spins, we can observe different "isomers". These isomers result from parallel or antiparallel coupling of the nuclear spins originating three or one quantic states respectively.
At room temperature, these different quantic states have the same probability, but the spontaneous transitions between them are extremely rare and can be ignored. As a consequence, normal hydrogen behaves as a mixture of three (volumes) of orto hydrogen (parallel spins) with one (volume) of para hydrogen (antiparallel spins)
For simmetry reasons, the rotational states allowed for the considered molecule differ in both cases, being the lowest energy state that of the para form.