Digestion begins before food even reaches your stomach. When you see or smell tasty food, your mouth waters; this liquid is saliva, made by glands under the tongue and near the ears. Saliva makes food soft and slippery so it is easy to swallow, and it contains an enzyme called amylase that starts breaking starch into sugar. That is why a piece of plain bread or boiled rice slowly tastes sweet if you chew it for a whole minute. Your teeth do the mechanical work: incisors cut, canines tear, and molars grind food into a paste.
The tongue is a muscle covered with thousands of tiny bumps containing taste buds. Taste buds detect five basic tastes: sweet, salty, sour, bitter and umami, the savoury taste of meat, mushrooms and fish sauce. All parts of the tongue can sense all tastes, though some areas are a little more sensitive. What we call flavour is mostly smell: when you have a cold and your nose is blocked, food seems dull because the smell of the food cannot reach the receptors high inside the nose. Taste also protects us; bitter and very sour tastes warn that a food may be poisonous or spoiled, and children are especially sensitive to bitterness.
Example: Blindfold a friend and ask them to pinch their nose shut. Give them small pieces of apple, raw potato and onion one at a time and ask them to name each food. Then repeat with the nose open. Prediction: with the nose pinched, apple and potato will be very hard to tell apart, because most of flavour comes from smell, not from the tongue.