Why does Clarisse rub a dandelion under montages Chin

Clarisse rubs a dandelion under Montag's chin because of the superstition that if it rubs off you're in love. She's playing with him..... 

"What a shame," she said. "You're not in love with anyone."

Ironically, the game implies that Montag is not in love....... he has to defend that he is based on the fact he is married.