Because the surface of the material is now electrically charged, either negatively or positively, any contact with an uncharged conductive object or with an object having substantially different charge may cause an electrical discharge of the built-up static electricity: a spark. A person simply walking across a carpet, removing a nylon shirt or rubbing against a car seat can also create a potential difference of many thousands of volts, which is enough to cause a spark one millimeter long or more.