I don't quite understand what a "spin" is. My dictionary has two definitions. 11. PHYSICS angular momentum: the intrinsic angular momentum of an elementary particle or system of such particles independent of its motion. 10. PHYSICS quantum property of angular momentum: the quantum property or number of an elementary particle that is a measure of its intrinsic angular momentum and magnetic moment. Is there a difference between the spin of a quantum particle and the spin of an atom, or combined atoms in a He3 BEC, or combined electrons in a superconducter? I would think an atom couldn't fall into the same category as a quantum particle or a photon at that. Quantum bosons are said to be force-carries, are other bosons force-carries too? Also, what can or cannot have a spin? I thought only quantum particles had a spin at first. And then I found out that atoms can too. And then I found out that two atoms together can have a spin. But I guess that is only at really cold temperatures, why can't atoms combined in a solid have a spin? Why doesn't a solid itself have a spin? What is that largest thing that has had a spin before, just two atoms connected together?