I mentioned the use of English, complicating billiards. (Brits call this 'side'.) Side English complicates the game a lot, and makes many shots possible which would not be possible otherwise. The main effect of side English is the angle that the cue ball (and to a lesser extent, other balls) bounces off the rail. The second most important effect of side English is squirt; the cue ball does not go straight in the direction that the cue is pointing, it squirts off to the side a little. Left English makes the ball squirt to the right. The greater the side English, the greater the squirt. And squirt causes you to miss shots, and is the main reason that the pros tell you not to use side English unless you have a good reason. The third most important effect of side English is Throw, which is the effect that cue ball spin has on object balls. There is also accidental side English, caused by a bad stroke. And other balls gain side English, mainly by banking off rails (but also because of throw). The point being, this isn't just deterministic pool ball physics: it's very much about having a 'feel' for the game.