Surprise!
Katara was hoping no one noticed that she dropped dessert down her dress. She excused herself from the table and hurried into one of the palace's alcoves. When she felt sure that no one was looking, she started digging.
"What are you doing?"
She screamed and turned to find Zuko peering in at her.
"I dropped dessert down my dress."
When he found it, he popped it in his mouth.
"What? I've eaten dessert off you before."
She giggled when he started licking the melted chocolate off her cleavage.
"Whoa…"
That's how Ty Lee found out about Zuko and Katara.