Merlin is a puzzle game. The window contains nine buttons each of
which can either be in the "pushed in" or "popped out" state.
Initially the buttons are pushed in or popped out randomly. The
player must clear the board by toggling the buttons until they are
all pushed in. Buttons are toggled with the mouse or via the appropriate
number key. The user starts the game with a number of points. Toggling a
button costs a point.  If you clear the board, you get some more points
and a new level to solve.  If you run out of points, the game is over.
Merlin records the high-score and the high-level between games.

WWW: http://www.doc.ic.ac.uk/~np2/software/merlin.html