Bridge Builder is a freely downloadable game, with the objective to complete each level's challenge by building a bridge strong enough to allow a train to pass over it without collapsing. Learning how the game works takes no time at all due to the incredibly simple control scheme. But figuring out what constitutes a strong bridge design can be frustrating, enlightening and exhilarating. There's a certain amount of fun to be derived from watching your painstakingly built bridge collapse unceremoniously, so even when you're frustrated, you're having fun. Thankfully, the stress analysis mode (which turns healthy beams green, and stressed beams red) can help you to determine where the weak point in your design lies.

This game is incredibly fun and addictive, plus it's also educational. There is a free version, but if you complete it and find that you're completely addicted, there are 3 newer commercial versions that you can play.