"Well, you're a little boy. You don't really get big muscles until you're grown up."
"Well, after puberty."
"Well, when you're a teenager, you'll start to get a bit more hairy. You'll need to start shaving because you'll get a bristly chin, and you'll grow hairs under your arms, on your chest, and around your willy. And it's only then that you might get big muscles."
"AND SOMETHING ELSE, MUMMY! BIG TESTICLES!"
"Er...well they're like little pouches of skin."
"And they look like little balls?"
"I've thought of a way I can see my testicles better!"
"Oh yes? How will you do that?"
"CHOP MY WILLY OFF!"