More than five
The idea of five senses goes back to the Greek thinker Aristotle, more than 2,300 years ago, and it stuck: SIGHT, HEARING, TOUCH, TASTE and SMELL. Scientists today count many more. There is balance, controlled by tiny fluid-filled loops inside your EARS. There is the sense of where your body parts are, which lets you touch your nose with your eyes shut. And there are separate senses for heat, cold and pain, plus signals from inside you such as hunger and thirst.
Fingertips that feel almost nothing
Your SKIN is packed with different kinds of sensor, and your fingertips have more than almost anywhere else. In 2013 a team of scientists in Sweden found that people could feel the difference between surfaces whose bumps were just thirteen nanometres high, far thinner than a human hair, simply by running a finger over them. That is how you tell ROUGH from SMOOTH, and every other TEXTURE.
In 1824, a fifteen-year-old French boy called Louis Braille, blind since an accident at the age of three, invented a system of raised dots that could be read with the fingertips. Each letter is a pattern of up to six dots, small enough to be felt with one touch. Braille is still used around the world, on books, lift buttons and medicine packets.
Eyes, ears and a hole in your vision
Each of your EYES has a blind spot, where the nerve carrying messages to the brain leaves the back of the eye and there are no light-sensing cells at all. You never notice it, because your brain fills in the gap using what is around it and what the other eye sees. Close one eye and move a dot slowly across your field of VISION, and at the right distance it vanishes.
Your ears turn the vibrations of a SOUND into signals, and they never switch off, even when you are asleep. That is why an alarm clock works. Our light and sound page covers how light and sound travel to reach you.
Taste is mostly smell
Your TONGUE can pick out SWEET, SOUR, SALTY and BITTER, among other tastes, and its taste buds wear out and are replaced every couple of weeks. But most of what we call FLAVOUR is really SMELL, detected by the NOSE as air from the back of the mouth rises into it. Hold your nose and a piece of apple and a piece of raw onion can be hard to tell apart.
Smell is closely linked with memory, because smell signals travel to parts of the brain involved in memory and feelings more directly than the other senses do. A SCENT or an AROMA can bring back a place or a moment from years before in an instant. Babies are born with a keen sense of smell and can recognise their own mother's scent within days. A few people have synaesthesia, in which one sense sets off another, so they may see colours when they hear music or taste flavours when they read words. Our food page covers a fifth basic taste that scientists added in the 2000s.