Not all guitars *do* have six strings. There are lots of special guitars that have differing numbers of strings. But, probably six strings is most common because it gives you a decent range and yet isn't more than most people can handle (and probably fits well into your hand for fingerings and such). But hopefully you'll have some guitar player who reads your blog and can come up with a much better answer! My daughter is too young to ask questions, but sometimes when I'm walking along with her and I'm bored, I'll start telling her how pendulum physics explains the movement of her swing, or why it matters what the angle of a slide is. Practicing for when she gets to Maria's age. :)