MOTM water buckets

Ok, I got to the room with the sand floor and the stone that drops when you stand on it and the water buckets. I cannot go any further because I can't figure out what to do before I die. I have tried different combinations of dumping out the water but cannot get it right. I hate it when I don't have time to figure it out, please has anybody got past this, spoilers welcome.....thanks for any help.