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.
This crunching noise is different to the fine crackling noise on neck movement you may hear after such a treatment. When a joint is stiff, the gristle loses water. On moving again, this water returns, over a few hours. Until then, the movement may be noisy. Value of neck muscle stretching