一道小学五年级的英语题

一道小学五年级的英语题
I don't have a mouth .I can sing songs .I have no muscles. I can be very strong. When I'm moving I shout loudly . when standing still I'm nothing
What am I?_________
急!急!
急！急！急！急！急！急！急！急！急！急！急！急！急！急！急！急！急！急！急！急！急！急！急！急！