Count the bicyclists, click anywhere for a small rotation, count again... WHAT!
What: This is one of famous puzzle makers Sam Loyd’s vanishing puzzles, Sam Loyd  himself is one of the most historied and famous puzzle makers and has published hundreds of thousands of puzzles. This one, the Disappearing Bicyclist is one of my favorites, using a simple rotation to hide the secrets of the puzzle, yet show you in full plain sight how it is operating.
How To: This one is relatively simple to operate, count the bicyclists on the screen, and then click anywhere and count again, you’ll see that they go from 13 to 12, and hence the conundrum will arrive… where did the extra come from (or go!?)
Explain it: In this case, while it’s tricky to see (especially in the analog format), the boys are slithered away each by a little piece, they all will seem a little smaller, and when you do the full count, they’ll be no more. A nice way to approach it is by looking at and counting the flags as well.
I've researched these illusions in my spare time but am clearly not any kind of expert and my explainations are pretty smooth brained, if you find something mis-cited, earlier examples, or general mistakes please new let me know via email@example.com, be kind!