Spot the difference

Spot the difference is a type of puzzle where players must find a set number of differences between two otherwise similar images, whether they are illustrations or photographs that have been altered with photo manipulation. Spot the difference games are also known as photo hunt games, and are commonly found in activity books for children or in newspapers.

The solution to the puzzle is often listed nearby or in an accompanying answer page of a puzzle book.

Additionally, one can cross one's eyes and merge the two pictures into one, in the much the same way as when viewing an autostereogram picture. The differences will appear to blink in and out of one's vision. This is a very effective method of solving these puzzles.

...
Wikipedia