I like to let people roll. The result may determine the consequences of failure.
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But then you get players who complain that "they couldn't even succeed with a 20?!" And they complain why did you let them roll.
Of course if your players are used to that party of your style, that's fine.
- Why did you let me roll???
- To see how loud the bystanders will laugh, obviously.
I've heard "because I like the sound of dice hitting the table."
No, you cannot attach a drop to this castle and drag it across the countryside, even with a nat 20.
Look, I'm not gonna tell you the DC so you can just make a note and come back later knowing you can pass. You're gonna have to see what happens and figure it out. The only time I will ever tell a player they can't do something, is if they are trying to do something that is against the rules of the game. I might inform them of potential consequences of their actions if they don't seem to quite understand something that should be obvious, but that's about it. I also use critical fail and success with skill checks in my games, to make things more interesting. So even if the DC is impossible, a NAT20 may still get you through.
I had a dm who would tell us to roll. And then say "you fail" before hearing the number