Yes, gravity. And how wide everything is. You can't stand outside up there, you've always got walls. Maybe it's infinite behind them, but you always know they're there. Down here you're just... out all the time. Nothing has boundaries.

[He waves his hand toward the lake, indignant, and then suddenly seems to remember what Skyler was actually talking about.]

Gravity. Yes, you all envy the birds. But you were born stuck to the ground, you're built for it. You don't have to think about it all the time. Maybe you don't want to be here, but you don't feel it.

[He's rambling, and some of it's not entirely coherent, but it's not like he notices.]