You could say something along the lines of:
"Hey, Friend, what's so good about weed? Why are you always smoking it?"
If he's smoking it for medical purposes, then just drop it, but if it's illegal and he's doing it to "look cool", then I would further ask questions about it.
Answer: C. Melatonin.
Explanation: Melatonin is the hormone responsible for sleep and wake cycles. :)
Answer: the first one is "lymphocytes"
the second one is "phagocytes"
Explanation: got it right.