A picture with a missing piece as a hole is presented to the user. The missing piece is floating in front of the picture that can be dragged with the mouse (or via touch). A slider let the missing piece to be dragged left or right, to make it easier than 2D free movement. The user has to drag the missing piece on top of the hole on the picture to prove he's a human. The location of the missing piece is randomized for each interaction.
