In reply to Denni:
Star Jumps.
Picture this - a very large military gym in 1983 with 249 people in nice smart red T-shirts, blue shorts, green socks and white plimsolls all doing star jumps in time. Except that in the back row is Number 250, your correspondent, doing something peculiarly different.
A large, PTI Sergeant makes his way over to this unfortunate specimen and bellows in his ear "Are f******g taking the piss, Sir, or what?" "No, Sergeant, I am not. I am trying my hardest, really", I pant. I'm not quite sure what he said next, but it wasn't complementary, but I got the gist of it.
Still can't do them, which the kids have always found immensely amusing.