I was so eager to try it out though, that I gave no thought to printing the sentiment before I decorated the card front. The inside should read "Have a ball on your birthday" or something to that effect (yes I know it's corny, but that's me).
Now I'm confident penny slider cards work with open ended tracks, more intricate cutting patterns can be used - the possibilities are endless. I'm thinking of a ship on the high seas or maybe something falling down some a set of stairs.
I've added a video so you can see the card in action. These cards should really be called penny twisters or penny spinners. The image doesn't just slide, it twists and spins - beautiful to watch and great fun to play with (though you really need 2 hands to keep the ball in motion and I only had 1 as I was holding the video camera with the other)