Call it a sixth sense or call it rocks in your head they're both right. How tiny stones inside our heads tell us which way is up on this Moment of Science.

Evolution To Balance

Smell, taste, touch, sight, and hearing: each one brings us a different kind of information about the world around us. But the oldest sense of all may be one evolved millions of years ago to help early multi-cellular organisms detect gravity.

We still have that organ in the form of a small bowl lined with tiny hairs inside each ear. Each bowl holds a thick fluid and tiny calcium stones a tenth the diameter of a human hair. When your head is upright, the fluid and stones cover the hairs in the bottom of the bowl. If you tilt your head, the stones and fluid slosh in that direction, bending the tiny hairs as they go.

Finding Direction

Those tiny hairs are connected to nerve cells, which tell the brain which hairs are being bent and in what direction. With that information, the brain can tell whether your head is upright, leaning to one side, or hanging upside down. In fact, this system is so sensitive that it can tell if your head is tilted even one degree to the side.

The system works more automatically than our other senses, so as long as it's working, we don't even notice it: we just

feel a little off balance and our muscles automatically set us right again. A bad ear infection can affect this tiny system and that's why earaches are sometimes accompanied by a loss of balance.

Gravity

Like our other senses it provides important information about the world around usso important that more animals have a sense of gravity than any other sense.