Why do we cry when we're sad? - Fun Kids - the UK's children's radio station

Here’s what Professor Hallux has to say:

Of course we can cry when we’re happy too! Our tear glands – or lacrimal glands – are constantly producing tears to keep our eyes moist and help clear away dust and other stuff we don’t want in our eyes, but crying is something that happens when we have all sorts of strong feelings or pain and as you probably know, it’s quite hard to stop yourself doing it, especially once you’ve started.

Why we do it is rather complicated and scientists still disagree on the reasons. Some people think that because it’s hard to see when you’re crying, it is a biological way of showing that your defences are down. In nature, animals do this to show others that they are not a threat – like a timid dog might show his tummy to show that he doesn’t want to fight.

It also could be a signal to others that you need help. Another theory is that because emotional tears contain certain hormones, it could be a way that the human body releases stress. And that might be why lots of people say that they feel better after a good cry!