This game
When you press start, the picture is broken into tiles that are semi-randomly arranged in a grid. One piece is missing. Click on a piece above/below or left/right of the missing piece. The piece you clicked will slide into the empty spot. Continue until you have all the pieces in order, and you will be rewarded with the missing piece. Yeah!
This page
I had a couple of these slider puzzles when I was a kid. They had superheroes on them. I thought it would be a fun/suitably difficult challenge to recreate the puzzle while teaching myself javascript.