On Monday morning in the office your colleague tells you that over the weekend he won millions in the Lottery and that this is his last week at work. You can’t believe it and so to make sure he isn’t joking, you ask him:
Are you pulling my leg? = jemandem einen Bären aufbinden
In an extension of this idiom, if somebody tries to trick you, but you are too clever for them, you can tell them to „pull the other one.“
The origin of this fun idiom is anything but fun. It joined the language at a time when London was full of thieves who attacked their victims by pulling them to the ground by their leg.