Relevant

In automatic models, when a user inhales through the device, air flow is detected by a sensor, which activates a heating element that vaporizes a nicotine solution stored in the mouthpiece. On manual models, the user must depress a button to activate the heating element to produce vapor. It is this vapor that is inhaled by the user.

On most models an LED on the opposite end of the device is also activated during inhalation, which serves as an indicator of use. Color of the LED is usually a matter of preference, as the most widely used electronic cigarettes come with options for many different colors.