Priest Gremory:
Then we understand each other. Go to the Room of Splendor and pour water into the Holy Grail. Then pour that water back into a jar and bring it to me. Thus can we verify the genuineness of the Holy Grail. Be careful, though; the Grail may be guarded.
One more thing: the Room of Splendor is locked. You will need a key to enter there. Some monks have such keys to perform their duties, so I suggest that you overcome one of them and take his key.