Molecules in the air dissolve in mucus inside the nose and are detected by olfactory receptors that send signals to the brain. In primates, including humans, there are two pathways. One goes to the olfactory cortex, the other passes to the hypothalamus, which is involved with emotion, motivation and memory. This part is responsible for whether we like or reject a smell. It may also be why smells and memory have a close association. Although humans have a comparatively weak sense of smell, it remains important. Babies who are just a day old show expressions of disgust when they smell fish or rotten eggs, while children can distinguish between the smell of their siblings and other children of the same age. Similarly, a baby recognises its mum’s smell and a mother recognises their baby’s. Even the humble fruit fly has complex olfactory processing. It has one system that identifies a smell and another that categorises smells as good or bad.