So make the ground height a variable. You don''t really need collision detection, just make sure the frog stops going down when his height matches the lillypads height. Sure, this isn''t an elegant collision detection system, but if you wrap it up in a couple classes (or just one) and use something like frog->groundHeight and stuff, it will be fine. And bugs in your implementation will be easily seen, cause the frog will start going through the ground, or into heaven... Hope that helps.