Taken from the Mythical Monster’s Storytelling Guide
Use the word ‘No’ carefully. No is a powerful word; It is absolute and often causes abrupt ends in conversation. There are of course exceptions to the rule but more often than not, there are better ways of signaling this to the players. Even if a player comes up with the most outlandish idea you have ever heard, offer them insight by putting it into the context of the situation. If a boulder blocks the path of the party and one character wants to punch through it, one way to respond might be “Okay. You can try to punch the boulder, but there’s a good chance it will have little effect and you may hurt your hand”.