"That's way cooler anyway." Rex thinks space is cool and hasn't grown out of it; also, he would notice that pistol if his eyes weren't otherwise occupied.

"... that makes me feel a little better." Don't tell César he said that. "Dude, yes, please. My nanites mean I'm always hungry!"

Which means he tries to hurry down the hall too fast before Crichton can guide him and hits the door frame with an "owwwwwwwwwwwwwwww!" but continues into the bathroom nonetheless.