Junk food, particularly sweets cause tooth decay, which causes cavities.  If cavities are left unfilled, they can cause further damage to teeth roots and even gums.

No! But I don't know anyone who really likes to do that anyway. Bleck!

Yes, but be VERY careful! That stuff burns if you leave it on too long and you definately don't want to get it into the wrong place!

They say they don't care, but I think they do. Maybe leave a little something. Not completely gone, if you know what I mean.