Count the number of people before and after the shifts in the image. What's the trick? It takes a few seconds before the change. Source and explanation of this specific puzzle.
There are some other examples of the same vanishing puzzle concept.
A circular version of this type of puzzle, called Get off the Earth was patented (in 1896) and popularized by Sam Loyd. He also made a version called Teddy and the Lions, inspired by Teddy Roosevelt. For more Sam Loyd puzzles see his cyclopedia of 5000 puzzles available on his (posthumous) website.