A spark is created by striking metal against a flint, or by pressing a button that compresses a piezoelectric crystal (piezo ignition), generating an electric arc. In naphtha lighters, the liquid is sufficiently volatile, and flammable vapour is present as soon as the top of the lighter is opened. Butane lighters combine the striking action with the opening of the valve to release gas. The spark ignites the flammable gas causing a flame to come out of the lighter which continues until either the top is closed (naphtha type), or the valve is released (butane type).