"It Only Works In The Dark" joke

A parish priest calls the Mother Superior into his office and says, "There is something I must show you. Please come into my private room and close the blinds."
"Father!" exclaims the shocked Mother Superior. "What did you say?"
"What I said was... " the priest begins.
"I heard what you said... I just can't believe you're saying it," interrupts the Mother Superior.
"Well, I really need you to come in," the priest says. Curious now, she does as he asks.
"Here now, sit on the bed beside me," he says.
"I must get out of here," the nun replies.
"Aren't you the least bit curious," asks the priest. She is, so she cautiously sits down beside him.
"Now get under the covers," instructs the priest.
"I can't do that!" she replies.
"But it doesn't work otherwise," the priest says. After much coaxing, she gets under the covers with him.
"Now, come closer," he whispers. Nervously, she moves closer to him.
"See, my new watch does glow in the dark!!" he whispers happily.