They trade positive rating and ad views for a hint, then sell a limited number of addition hints. The random appearance of ads is annoying and interferes with game play.
Some of the puzzles are not well thought out; they could learn a few things about puzzle construction from Cubic Room, which is a delightful game series I recommend with honest enthusiasm. Plus, Cubic Room trades in-game ads for hints, which is a much more pleasant way to monetize an app than interrupting the game at random with ads.
Cubic Room is fiendishly clever, has entirely fair if sometimes brain-busting puzzles, boasts great graphics in a reasonably interactive environment, and makes completing chapters feel like a reward rather than a release from the Gulag.
I would pay for a whole series of Cubic Room games. Wouldn't pay a dime for this one.
I told you to go get the Cubic Room series, right? Okay, my work is done here.