On the PHP-Tip-a-Day site there's a new post from Greg Bulmash with an allegory about the Factory pattern (design pattern) to relate it to something a bit more practical and introduce some of its core concepts.

If you've read the About This Site page, you'll know that instead of pursuing my passion for computers, I got sidetracked into the arts in college. So when it came time for me to learn and explain the Factory Pattern, I thought it might be fun to express it as an allegory...

His example involves cars, axles and wood to illustrate how the Factory pattern lets you generate objects of different types automatically without having to worry about how they're created. He includes a code example in the form of an interface and a class that implements it (the "Axle" class).