E-liquid (also known as e-juice) is the substance inserted into a vape pen which gives vapour its flavour and nicotine (if the vaper is using nicotine e-liquids). When the vape pen is in use, the battery will provide the energy to heat up the e-liquid, turning it into a smooth vapour when inhaled. Offering complete versatility, e-liquids allow vapers to experiment with e-liquid flavours in a choice of non-nicotine or nicotine strengths.