If I was caught red handed I would say: "I'm smoking pot in order to cope with the 30 dead children in my basement..."

But seriously though it depends on how busted I was. If I was in the process of smoking a joint or something and my parents literally just walked in on me puffing it, then the only excuse I could come up with is that it helps with my art projects in school and is the reason why I have a 3.5 gpa.

But if I was caught because my eyes were red or because of the smell I would definitely deny deny deny by coming up with plausible excuses like "my eyes are red because I just cried and I'm going through a lot right now which I'd rather not discuss".