Pull somebody’s leg
Definition 1 of 1
Pro Tip 1/3Informal Use
Best suited for casual conversations among friends or in relaxed environments, not formal settings. 
I can't believe you fell for it. I was just pulling your leg!
Pro Tip 2/3Humorous Intent
Intended to be funny; the speaker means no harm when teasing someone with this phrase. 
When Mike said he met a celebrity, he was pulling your leg. He's known for his jokes.
Pro Tip 3/3Reaction Check
Often used to see how someone reacts to false or exaggerated information as part of the joke. 
Are you really moving to Mars, or are you pulling my leg?