Hello, I am a new DM and as I was reading the DM guide I had a question regarding the rolling buolder trap and perception. It says the on a perception check of DC 10 a player notices the approaching boulder. The question is when does this check happen? Is it as soon as the trap triggers or should i make a passive perception check as soon as they enter the room? Also does the perception check repeat while they are in combat? i figure that if they are in teh heat of battle they might not notice the boulder coming. and the last question is if they fail the perception check, then what happnes? ANy information would be useful and help appreciated.