Sometimes, when you are walking over a carpet, your shoes pick up extra electron from the carpet when they rub against the fiber. These extra electrons give your shoes an electrical charge. Now, if you go and touch something like a metal doorknob, the electrons are discharged, and you feel a little shock. If the room is dark when this happens, you may see a blue spark. The spark is caused as the electric charge jumps from your fingers to the doorknob, before you actually touch it. The blue color comes from the nitrogen atoms in the air. All types of atoms have their own characteristic color, and nitrogen atoms are blue when they become ionized, are electrically charged.