DID you play a harmless joke on your best friend last Wednesday, on April Fool's Day? On that day, people across the world play funny jokes on others. Even the media plays along. In the UK, the Express announced a new advance: a plastic film that when wrapped (包裹) around a car, makes it invisible (隐形的) to speed cameras. And the BBC, which discovered a flying penguin last year, broadcast wildlife footage (镜头) of a never-before-seen “shark-whale” – an animal that is half shark and half whale.