That can get people hooked initially. The tricky part is to get people coming back. That involves increasing the difficulty (but not too much), so the user fee

That can get people hooked initially. The tricky part is to get people coming back. That involves increasing the difficulty (but not too much), so the user feels a sense of accomplishment. Throw in elements of randomness and, voila, you have an addictive game. There are studies that go into more depth than this, but that's the basic outline.