Scaling Web Applications & Tower Defenses
If I were to try to teach a 15-year-old how to scale well on the Internet, I would let him play some Warcraft III tower defense map.
The analogy is pretty striking really: you have a more or less constant stream of visitors (creeps), and your goal is to not only meet the performance needs (deal enough damage), but also one of efficiency: how much gold do you need to deal enough damage to kill all the creeps -- or how many dollars do you need to perform well enough to handle all the traffic.
I'm not going to tell you how, because the TD analogy also lifts an interesting aspect: unless you know how the game works and what the alternatives are, you're going to suck at it. Perhaps less than some, probably more than others, but you're going to suck nonetheless.
Thought I'd let you know.