[ Castiel follows, too distracted by the turn of conversation to really contemplate the logistics Bruce is imposing upon him. He just follows - the one thing in life he's good at. Or rather... he's better at it then at making his cown choices, and sometimes despite all that he's fought hard for free will, following is still a comfort he all but craves.

So for now, he does. Bruce leads. Castiel trails after. Gets momentarily distracted by something on a side table, picks it up and observes it before putting it back and following - if Bruce watches on their way, he will notice that sometimes. The way Castiel likes to look at things, intoxication writing the wonder clearer onto his features. It's not childlike - he does understand what things are and what their purpose is - for the most part. Still.... humanity fascinates him.

Angels do not produce art, they don't dream, they aren't even designed to feel.

And yet... and yet.

Castiel never lets himself fall far behind. Though sometimes he obviously gets distracted a little more, and has to close the distance with a soft flutter of wings. He manages not to fly into walls either, which Castiel personally counts as a win.

And after a while, the answer to the question comes, slightly hesitantly because he has a feeling that his answer won't quite satisfy the criteria of the question: ]

I don't require sustenance. Which is to say, I can eat, I just typically don't. Most things just taste like molecules.

[ He's been enjoying coffee though, which Bruce will be aware of - coffee dates with Riku were originally how Castiel "smoothly" justified checking in on the inhabitants of the museum, after all. ]