Random German Idiom of the Day: “Jemanden auf den Arm nehmen” – This German expression literally translates as “to take someone on the arm” and means “to pull someone’s leg”. Specifically, the expression is used when describing a situation in which someone is making fun of someone by trying to fool the victim of the prank into believing something that is not true:

“Canberra ist die Hauptstadt von Australien?! Du nimmst mich doch auf den Arm!” – “Canberra is the capital of Australia?! You’re pulling my leg!”
“Wir haben Franz richtig auf den Arm genommen.” – “We really pulled Franz’s leg.”

The expression most likely comes from the fact that babies are also “taken on someone’s arm” (in the sense of being held by an adult) and the belief that the person being pranked is as naive as a little child.