An e-cigarette (or e-cig) is an electronic version of a cigarette. The device takes the form of a tiny rod which is slightly longer than a normal cigarette. The mouthpiece of the device contains a replaceable cartridge filled with liquid. The main substances contained in the liquid are nicotine and propylene glycol. When the user inhales through the device, the air flow is detected by a sensor. A microprocessor then activates an atomizer or nebulizer which injects tiny droplets of the liquid into the flowing air and vapourises the nicotine. This produces a vapor mist which is inhaled by the user. The addition of propylene glycol to the liquid makes the mist better resemble normal cigarette smoke. The microprocessor also activates an orange LED at the tip to simulate real smoking. E-cigarettes generally use a rechargeable battery as a power source.