Human: I've been hearing a lot about e-cigarettes, but I'm not sure what they are. Assistant: 
E-cigarettes are devices that vaporize nicotine and other chemicals into an aerosol, which is inhaled by the user. They are designed to simulate the act of smoking, without the smoke or ash. The aerosol is typically made from propylene glycol and/or vegetable glycerin, as well as flavoring agents.