49 BLUE ROOM. The door to this room is painted blue while the interior walls, ceiling, and floor are painted light and dark blue with a swirling pattern.
The shallow fountain contains clear odd-smelling liquid and a small blue key. When the door to the corridor shuts, the stopper on the bottom of the fountain will be pulled out and the key will soon be sucked down the drain. Give the characters about 1 minute in real time to choose how they want to get the key, a stopwatch may prove useful here.
The trouble is that the liquid in the pool is actually clear acid. A splash will cause a point of damage, while inserting a hand or other body part will cause 2-5 points of damage. Drinking will cause 2-8 points of damage and requires a saving throw versus poison if death is to be avoided. Weapons or other objects dipped in the acid are entitled to a saving throw, see page 80 of the Dungeon Master's Guide.
There is no reason that the players can not use the acid for combat or other uses if they have a container that the Dungeon Master decides is adequate, but the fountain will not renew itself. Rules for using acid as a weapon are on page 64 of the Dungeon Master's Guide. Several gallons of the stuff are present. The blue key is itself impervious to the acid, and shows no sign of harm.
Assuming they manage to get the key, it will open the locked blue door to Room 53 together with the red, yellow, and green keys. Only the keys can open this or the other doors to Room 53, although multiple knock spells will work for this door or the other doors leading to Room 53. One knock is required for every lock on the door for which a key is not possessed.