One professor noticed people often say that, despite their blotchy faces, a good cry makes them feel better. So he did an experiment where people had to breathe in over a blender full of onions that had just been chopped up. Not surprisingly this made their eyes water. He collected the tears and put them in the freezer. Then he got people to sit in front of a very sad film wearing special goggles which had tiny buckets hanging off the bottom, ready to catch their tears if they cried. The people cried, but the buckets didn’t work and in the end he gathered their tears in tiny test tubes instead.

He found that the tears people cried when they were upset contained extra substances, which weren’t in the tears caused by the onions. So he thinks maybe we feel better because we get rid of these substances by crying and that this is the purpose of tears.

But not everyone agrees. Many psychologists think that the reason we cry is to let other people know that we need their sympathy or help. So crying, provided we really mean it, brings comfort because people are nice to us.

Crying when we’re happy is a bit more of a mystery, but strong emotions have a lot in common, whether happy or sad, so they seem to trigger some of the same processes in the body.