Anything that moves someone emotionally — hearing a baby say their first word, finishing a feel-good TV series that you wished had never ended or reading a breakup text — can bring on tears. In fact, according to the American Academy of Ophthalmology, the eyes produce 15 to 30 gallons of tears a year. Although animals cry, too, humans are likely the only ones that shed tears for emotional reasons. But crying is more just than a way of expressing sadness or happiness. There are actually different types of tears. So what are they and why exactly do we cry? Experts explain.