We produce three different kinds of tears. Basal tears lubricate the eyeball and make it optically smoother. These are produced continuously at a rate of almost 300ml a day. Reflex tears are produced in response to physical or chemical stimulus and wash away irritants. The third kind is produced in response to high levels of emotion. This stimulates the cranial nerve in the brain, which sends neurotransmitters to the lachrymal glands. It also raises the blood pressure in the face, which tends to increase the flow of tears. Emotional tears contain high levels of manganese and the hormone prolactin (which normally stimulates the mammary glands to produce milk). Humans are the only animals that cry in this way and it has been suggested that emotional crying may have evolved as a way to remove excess levels of certain chemicals and only later became ritualized as a way of signaling emotional distress.