Swollen shoulders

A conversation between No. 2 Son and myself at the YMCA one morning:

No. 2 Son: "Dad, I think there is something wrong."
Dad: "Why? What do you mean?"
No. 2 Son: "My shoulders, they're all swollen."
Dad: "Well, do they hurt?"
No. 2 Son: "No, they're just swollen."
Dad: "Let me see." Observes No. 2 Son's shoulders closely. "Dude, they aren't swollen, that's just muscle."
No. 2 Son: "Really? How?"
Dad: "Well, you know how we've been coming to the Y? And how we've been lifting weights? Well, that's what happens when you lift weights; you get muscles."
No. 2 Son: "Cooool."