[ Biff had noticed the guy having such a blast in the water earlier — it's not for him, water, he's had plenty of time for swimming in his life so far. So he'd just taken idle note and moved on. He's passing by a little bit later when he notices the same guy again, this time on the edge of the pool and looking like the weight of the world had just fallen onto his shoulders. That's a look Biff knows well.

It's almost against his will that he gravitates toward him, heading in from behind on quiet bare feet that, combined with the jeans-and-a-t-shirt look, really make him look like he'd just been yanked out of his own living room.

(It was a hotel room, actually.) ]

What happened? You looked like you were having so much fun splashing around in there I was going to ask if it was some fun new sin.