The molecule that immediately comes to mind when you think of cigarettes is nicotine.
Nicotine is the active ingredient of tobacco and is shaped like a neurotransmitter called acetylcholine.
Acetylcholine and its receptors are involved in many functions, including muscle movement, breathing, heart rate, learning and memory.
Nicotine works by attaching to the acetylcholine receptors in the brain and mimicking the action of this natural chemical.
Nicotine also raises the levels of the "pleasure molecule", dopamine, in the brain, bringing with it sensations of pleasure and reward.
A cigarette will contain about 1.2 milligrams of nicotine, but this is diluted when inhaled. It’s been claimed that if this amount of nicotine were injected, it would kill 7 adults.
There are more than 4000 chemical compounds created when you take a puff on a cigarette.
The following eleven compounds in cigarette smoke are known to affect human health: benzene (a volatile hydrocarbon, which causes lymphomas and leukaemia), 2-naphthylamine and 4-aminobiphenyl (aromatic amines, known to be human bladder carcinogens), vinyl chloride (an organic compound known to form tumours in the blood vessels of the liver), arsenic, beryllium, cadmium, chromium, nickel and polonium 210 (all metals and human carcinogens) and ethylene oxide.