(a) In the BEC limit, all the atoms are in pairs with different spins (blue and red balls) that are bound together into diatomic molecules (grey circles) that form a Bose-Einstein condensate. In some of the diatomic molecules, the pairs are very close together (highlighted boxes). (b) The unitary limit is intermediate between the BEC and BCS conditions. (c) In the BCS limit, a tiny fraction of the atoms form very large Cooper pairs (grey circles). A small fraction of the atoms are very close together (highlighted boxes).