Quote Originally Posted by SowZ View Post
If the best roll possible can't succeed, I will simply tell the player it is impossible for whatever reason. If I let them roll, I feel like I am admitting there is a chance.
Personally, I still let them roll. While I might know what their bonus to the particular skill is, and that the DC is higher than 20+skill level, I never know when they might find some way to boost their check. If I just said "no, you can't do it, period", that wouldn't necessarily be fair if they COULD make it after, say, getting assistance from another player, having the bard sing encouraging things to them, after quaffing a potion of Eagle's Splendor/Fox's Cunning/one of the other similar spells that boost your stats, and using a set of masterwork tools for whichever skill he's working on. Of course, they'd have to have a specific sub-set of skill-boosting items or abilities handy, but I know at least a few of my players do plan ahead that much. They'd still have to be ridiculously lucky, or spend a lot of time to work on it, but if there is some slim chance due to a combination of effects, I'll still let them roll.