Usually, we adults are surprised when the younger generation do know something we don't expect. But are there any times you have been surprised by a child or teenager being quite unfamiliar with something you thought they would know about, or have had to shift your expectations? Here are some I things I have had to explain unexpectedly, come and share yours!
Pin the tail on the donkey. I used it in a maths question about probability: a pupil had never even heard of it! I didn't get them to play it though. 
Also about probability, I've sometimes had to explain playing cards to teenagers who have never used them.
I once had to explain to a teenager what a microscope is used for: it was news to them.
I was going to add the Millennium Bug to this list (it seems like only yesterday everyone was terrified of the impending doom), then I realised most of my pupils hadn't been born then! 
I quite enjoy telling pupils about old technology, such as looking things up on microfiche, or TVs that were not flat screen.