We don't have a category for this - so thought I'd collect a lot of the stuff we have here, in one tidy place.
I've always enjoyed April Fools and use it as a way to "test" my students. See the example below - a trick test.
Also, I'll often show this video as if it is real. I've also changed everything around in the classroom and how I dress. Shoes untied, clock upside down etc... and then acted as if things are normal. Students have to tell me what is wrong!
This video is perfect to "fool" students. Flying penguins.