Chemicals affect us in different ways. Some, such as adrenaline and testosterone, are hormones produced by our own bodies, and control our physiological responses.

Other molecules we’re often exposed to come from outside our bodies, but can make us feel good. Some, such as perfumes, which are often in the air we breathe, smell delightful and react with olfactory receptors in the nose.

Some are drugs and make us feel good but can have adverse effects on our bodies - such as alcohol, caffeine in coffee and nicotine in cigarettes.

Caffeine is addictive, but gives us a buzz.

Alcohol can make us happy, but in large doses can do our bodies permanent damage. Nicotine can also bring the sensation of pleasure, but has a bad effect on our muscle movement, breathing and memory.