The Joker's Provokers (2) - S2-E22
Corrected entry: While Batman and Robin are going over clues, one of them is O-HO. Robin says it's the formula for water, which is H2O. The two is attached to the H, not the O. O-HO should have led to hydroperoxyl, which is HO2.
Correction: That's the point of the clue, to contain the information without giving it away. By saying out loud H two O's it leads Robin to say the formula for water. The H and O's in the clue don't represent hydrogen and oxygen; just like a clue with a sheep (ewe) represents "you".
Bishop73