Great question! The shock that you feel when your body comes in contact with metal is called static electricity. EVERYTHING including our bodies, door knobs, hamburgers, cats/dogs etc are made up of atoms. Atoms are also made of things called particles. Think of an atom like a solar system....the sun in the center of the atom that contains positively charged particles called Protons and the planets are negatively charged particles called Electrons. Just like the planets move around the sun, electrons move around the protons. In the case of atoms the electrons that are farthest away from the center of the atom can move from one atom to another. Since EVERYTHING is made up of atoms, that means the electrons in your hands atoms can move to the atom of a piece of metal. When this happens you have lost a negative charge and the metal has gained a negative charge. The transfer of charge from one object to another is what creates the shock you feel. It is also the same reason your hair stands on end when you rub it with a pillow.

If you want to read more about static electricity try this web site on "science made simple"