I guess I'll offer up the first legitimate answer to this thread:
I smoke , and as far as I know, my 18 & 16 year-old do not know that I do.
If they asked me directly, I would tell them.
Also, my 1st is a freshman in college - I did not want to be hypocritical, so all I told him is that peer pressure would probaby introduce drugs to him. I played it from the legal angle, stating that since he worked so hard to do well, he would have to decide if the risk of being caught was enough for him to say no. I never once told him to never do it, nor at this point have I offered up my smoking (but again, I would honestly tell him if he asked).
I assume (bad word) that my kids do not smoke, but I'm not stupid - they obviously could without me knowing.