For example, one of the Phoenix's more consistent obsessions is with Dante's Divine Comedy. So, the solution to one puzzle involves looking up Inferno on the web (in Italian, no less), then inputting the canto and line numbers of a verse flitting about the screen.
Not every solution requires that kind of work - some can be solved just by logic - but many do, and if you're the sort who has a lot of odd, historical and literary trivia floating around in your head, it's kinda neat to run across a game that rewards it.
It's worth noting, however, that other searches and clues point to web pages specifically designed as part of the game, and one kinda cool touch is that, unless you're paying very close attention, it's not always apparent which pages are real and which aren't. In addition, the game plugs you into the Phoenix-Investigators.org website, so you can get in touch with other players from around the world and trade ideas and solutions.
But as different and challenging as Evidence is, the P-I site also brings up the game's more annoying side. While the site has a rudimentary search function for finding other players to team up with, there's no apparent way to search for a specific player. So if you and a friend want to get together, you're kind of screwed (unless you're very lucky and a random search happens to pop up with his or her screen name). True, you can always communicate outside the game, but it's still pretty annoying.