Christmas With the Joker - S1-E38
Joker: Jingle bells, Batman smells. Robin laid an egg. The Batmobile lost a wheel and the Joker got awaaaaaaaaayyyyy.
Christmas With the Joker - S1-E38
Robin: OK, I'll make a deal with you. If we go out on patrol, and Gotham is quiet, with no sign of the Joker, we come back here, have Christmas dinner, and watch It's A Wonderful Life.
Batman: You know, I've never seen that. I could never get past the title. (00:02:20)
Answer: Because he stole a chemical from his lab.
But Batman knew that with it, Clayface could permanently retain his shape and Clayface desperately wanted to be Matt Hagen again. In a sense, Batman stopped Clayface from becoming human.