E-cigarettes are smoke-free and tobacco-free, but they're not nicotine-free. The liquid in e-cigarettes is typically a combination of nicotine, flavorings (such as bubble gum or watermelon), propylene glycol (a solvent), and other additives. The amount of nicotine depends on the mixture of the particular liquid-nicotine cartridge installed in the device. Some products contain nicotine amounts comparable to regular tobacco cigarettes, while others contain levels closer to that of a light or ultralight cigarette. There are also cartridges available that contain flavored liquid without nicotine, for users who want the sensory experience of smoking a cigarette without the harmful effects.