If they're working at a fast-food place, and you say the "right" words, i.e., what's on the script running in their head, then they're "OK". But if you deviate from the script, you can actually see, from the expression on their face, that their mind is short-circuiting. If it were a cartoon, there'd be little sparks shooting out their ears, with smoke.
To be fair, however, I've also met teenagers who are the exact opposite. Thinking, rationale, cognitive,... human.