how to tell male/female

Question
My friends nephews have new guinea pigs but noone knows how to tell male from female.....can you tell me how to distinguish one from the other? Thanks.

Answer
Hi Bren,
I am sorry for not answering quickly!
It is a bit hard to tell you how to distinguish a male from female, but I'll try!
The male would have a little "bump" at the opening and the female won't.
It' a bit hard to tell, sorry!
I hope this helps, If you have more quesions, dont hesitate to ask.
Amparo