Pull someone's leg

Meaning/Usage: Making someone believe something that is not true.

Explanation: This came from London long time ago where criminals would trip someone to make them fall. After they fall, another criminal acting like they are helping the fallen person would take valuables out of their pocket. Although pulling a leg is not tripping, it can be seen as falling away from the truth.

"You expect me to believe that? Stop pulling my leg."
"I'm just kidding. I was just pulling your leg."
"I didn't know you believed me. I was just pulling your leg."

A: "I was eating at a restaurant and this lady next to me started eating my food right in front of me."
B: "What? You can't be serious. Nobody would ever do that."
A: You're not gullible at all. I was trying to pull your leg."