Fifteen tiles, one empty square, and a picture cut into pieces. Slide a tile into the gap, repeat until the image is whole. That is the puzzle everybody has held in plastic at some point, and on its own it takes about five minutes once you know the trick of solving it row by row and leaving the last two columns for the end.
The reason to stay is the panel down the side. Load your own image, change the grid to something other than four by four, set how fast the tiles move, turn the numbers on or off. A three by three is a warm up and a six by six is a genuine afternoon, and the puzzle you assemble out of a photograph you chose is harder than the stock one because your eye stops helping.