TOM: Sam! Help me!
SAM: What's the matter, Tom?
TOM:: I'm hanging from this rock by one hand. I can't hold on much longer. Call the rescue service quickly!
SAM: Very funny, Tom. Why are you always pulling my leg?
TOM:: Sam! I'm not having you on. It's true! Honestly!
SAM: Pull the other one, Tom!
something untrue (for fun)
pull the other one
I don't believe you