Answer:
well ther is carbon diaxoide
Explanation:
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:
direct observation
Explanation:
because if your constantly monitoring them you should see any abnormal signs