Your inner ear, in addition to having all the mechanisms required for hearing, contains a very tiny vestibular (or balance) organ. This organ consists of three semicircular canals or hollow tubes connected to an aptly named vestibule.
Each tube detects when your head undergoes one of three of particular types of motion called "yaw", "pitch", and "roll". Yaw motion is when you move your head side to side saying "No". Pitch motion is when you nod you head up and down saying "Yes". Roll motion is when you head tilts from side to side, like when you're falling asleep on an airplane.
Each semicircular tube is partially filled with a water like liquid called endolymph. Floating on the liquid are very tiny particles of calcium (like grains of sand) called otoconia. The inside of the tube itself contains millions of microscopic hairs that jut out and are in contact with the liquid and calcium. Imagine a half full (or half empty if you're a pessimist) glass of water with a few ice cubes floating in it. Further imagine that there are lots of tooth picks taped to the inside of the glass that are jutting out like spikes into the water. That's about how it looks inside one of these semicircular tubes.
If you mentally tip the glass a little you will notice that the ice cubes tend to bump into the tooth picks on the all sides of the glass in different amounts. This movement also happens in the semi-circulars canals when you tilt your head. When it does, the hair cells send out impulses across your vestibular nerve and to the brain stem. Your brain miraculously interprets the signals and determines the position of your head.
