molecule. 1  Chemistry, Physics. the smallest physical unit of an element or compound, consisting of one or more like atoms in an element and two or more different atoms in a compound. 2  Chemistry. a quantity of a substance, the weight of which, measured in any chosen unit, is numerically equal to the molecular weight; gram molecule.
When I do crunches my neck starts to tighten up and it makes me have to rest before doing another set of crunches.
Cracking the neck joint (also called neck popping) or any joint actually leads to release of fluid or gas within the joints. It is an effect of a sudden development of temporary vacuum develops and is filled by gas, which was trapped in the joint fluid. Thus, when you stretch that joint, 'pop' or crack it goes.