The semicircular canals in the ear contain fluid and small stones (otoliths) that move with head motion, deflecting hair cells to send balance information to the brain.

“Those marbles within each one of those hula hoops can move around, but they'll only move around if your head moves in a particular way... And when they roll back and forth, they deflect little hairs, little hair cells... and causing them to deflect. And when they deflect downward, the neurons, because hair cells are neurons, send information up to the brain.”