The cartridge, a small plastic container with openings on each end, serves as both a liquid reservoir and mouthpiece. It allows the passage of liquid to the atomizer and vapor from the atomizer back to the user's mouth, without leaking liquid into the mouth.
Most models adopt a plastic sponge to keep the liquid in place, but it is also common to use a refillable tank to hold the liquid, with a separate tunnel connecting to the atomizer. When the liquid is depleted, users can refill it or replace with another pre-filled cartridge. Some users forgo the use of liquid reservoirs and drip liquid directly onto the atomizer in a method known as "dripping".