An electronic cigarette, also known as e-cigarette among other names, is a handheld battery-powered vaporizer that simulates smoking and provides some of the behavioral aspects of smoking, including the hand-to-mouth action of smoking, but without burning tobacco. Using an e-cigarette is known as "vaping" and the user is referred to as a "vaper. " Instead of cigarette smoke, the user inhales an aerosol, commonly called vapor. E-cigarettes typically have a heating element that atomizes a liquid solution called e-liquid. E-cigarettes are automatically activated by taking a puff; others turn on manually by pressing a button. Some e-cigarettes look like traditional cigarettes, but they come in many variations. Most versions are reusable, though some are disposable. There are first-generation, second-generation, third-generation, and fourth-generation devices. There is also pod mod devices that use nicotine in the form of a protonated nicotine, rather than free-base nicotine found in earlier generations. E-liquids usually contain propylene glycol, glycerin, nicotine, flavorings, additives, and differing amounts of contaminants. E-liquids are also sold without propylene glycol, nicotine, or flavors.