This was an update to my first game ever. It is pretty much as simple as a game can get, and was an excellent launching point to learn some basic coding. Oh, and it's TOTALLY XTREME.

[Single player] Click on-screen to click your pen.

[Player 1 (RIGHT) ] Home key clicks.
[Player 2 (LEFT) ] Enter key clicks.

Click your pen faster than your opponent.

Although all games can cause repetitive strain injuries, I'm just going to take a wild guess and say that you shouldn't play this particular game for hours on end.