我们为什么要哭?

It's obviously a very highly evolved behaviour because we are the only animals that do it. Other animals, although this is possibly disputed by some scientists, don't appear to cry. It's thought that you cry in response to a stimulus, some scientists thought it might be to do with the build up of stress and hormones - a hormone called adrenocorticotropic hormone might actually be released through the tears. So when you're in a stressful or unhappy situation, you actually release this hormone and get it out of your body through your tears. There is evidence for certain hormones found in your tears, things like the hormone prolactin, other things like potassium and manganese. And so, tears produced when you're crying for emotional reasons actually have more of these things in than tears that are just lubricating your eyelids. So perhaps, you might be able to tell if someone is just faking it or just the got onions out by measuring these hormones in their tears. Interestingly, another thing about crying, women do cry more than men. It's thought to be to do with certain hormones that are only found in women. As studies showed on average that men cry about once a month, women cry about five times a month on average. (More around the time of the month ladies.)

The other interesting thing about crying is that it may well have evolved as a communication signal to say, "I'm really unhappy and I need a cuddle" or "I'm really upset. I'm really angry. I'm really stressed." Because obviously, people can see your tears and respond to them, so it may well be some kind of signal - showing that you're vulnerable, that you're unhappy - that other people can respond to.