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.
Crunches can firm up the muscles under your fat and keep your core strong, and that's important for many reasons. But they won't make that fat magically vanish on their own. For fat erasing, you need regular aerobic exercise, such as walking.