"They make things out of petroleum," he explains vaguely. "Grown in the dark, covered in choking black dust?" This image tears him between amusement and melancholy for a moment, and then he asks, "What did he decide?"
He nods. "It means I don't have to worry about you falling over randomly."
"When would you have?" he asks wryly.
Not quite understanding the look, he lets it pass. "If they do come after us later. Just for a few minutes, because they don't have imaginations. And then maybe sneak a post-hypnotic suggestion in while they're confused..." One thing you can say for Snape: he has the face for Evil Overlord-type contemplative pre-gloating, and the eyebrow action to back it up.