Well, famously thinning follicles, a detail that the lushly maned Bieber seemingly had no problem mocking.
Hey, just because the heir apparent's hair is not so, um, apparent, is that any reason to make fun? Well, yes, as it turns out.
Justin's jibes, good-natured though they may have been, came out during an interview with the U.K.'s Rollercoaster magazine, with the Canadian (so much for sticking by the Commonwealth) singer offering up some advice to Kate Middleton's better half.
"I mean, there are things to prevent that nowadays, like Propecia," the 18-year-old said. "I don't know why he doesn't just get those things, those products. You just take Propecia and your hair grows back. Have you not got it over here?"
Ah, the brazen logic of teenagers. Prince William, who's back to work after enjoying the first week of the Olympics, has not and—barring the sudden appearance of snow in a certain scorching underworld—won't comment on the barb. A stiff upper lip never came in so handy.