When a person steps on something sharp (like a nail), the leg that is stepping on the nail reflexively pulls away (the flexor muscles contract and the extensor muscles relax), while the other leg reflexively takes the weight of the whole body (the flexor muscles relax and the extensor muscles contract) in preparation for the jerk.
A molecule is the smallest particle of a substance that exists independently. Molecules of most elements are made up of only one of atom of that element. Oxygen, along with nitrogen, hydrogen, and chlorine are made up of two atoms. Look at the model of oxygen above. The two balls represents the two oxygen molecules.
An atom is the smallest unit of matter.(well, there are smaller(quarks, leptons) but for the sake of simplicity, atoms are among the smallest) A molecule is something like H2O(water) and is made up of atoms. A cell is made up of atoms and molecules. Easy stuff brother.(or sister)