Why Do We Cry?

Step 1 of 5

Your eyes are wet every second you are awake.

A thin film spreads over the surface with each blink, carrying oxygen and washing away dust. Without it the surface would scar within days.

Step 2 of 5

The film needs all three layers.

Too little oil and it dries out in minutes. Too little water and it cannot rinse. Dry eyes are usually a broken layer, not a missing tap.

Step 3 of 5

Irritation opens the taps.

Nerves in the surface detect grit, smoke or onion fumes and trigger a flood far bigger than the film, to wash the problem away.

Step 4 of 5

Emotional tears are chemically different.

They contain more of certain proteins and hormones than rinsing tears. Nobody is certain why, but the difference is measurable.

Step 5 of 5

The overflow has to go somewhere.

Tears normally drain through tiny holes into your nose. A flood outruns the drain, so it spills down your face and makes your nose run too.

Crying is thought to be a social signal as well as a physical one: a visible sign of distress that other people respond to.