I have no idea. I didn’t even know until today that other animals menstruate! But it seems they do… well there you go. I’m a physicist so I really haven’t learned much biology! Sorry can’t help you on this one.

Girls have different sex organs, and produce eggs – if eggs don’t get fertilised, then the body gets rid of them. Boys don’t produce eggs (although they produce sperm), but they don’t need to ge rid of them.

Hehehe we aren’t the school nurse!
I can answer this for you though, and if you have any other similar questions I don’t mind answering them either.
Boys can’t have periods because they have male reproductive organs.
Girls have periods because: once a month girls ovulate, release an egg from the ovaries to the lining of the womb. In case the egg gets fertilised it needs somewhere nice and soft to live, so the lining of the womb builds up and up until it is thicker. If that month the egg doesn’t get fertilised the lining of the womb breaks away and it released from the body as a period. It is mostly made from blood so that is how we see it.
This means that when you are pregnant you don’t get periods, because the lining of the womb stays built up to keep the womb nice for the growing baby.

Women have periods as a way of regularly ‘resetting’ their biological system and making sure they are ready to have a baby. (I realise that this is going to be such a boy’s answer to this question!). Hormones trigger the lining of the uterus to shed as part of the reproductive cycle, which also involves a new egg moving from the ovaries into position for fertilisation.

You know this is the Space Zone, right?