Nothing made me happier as a kid then getting dirty while playing sports. Seriously, even to this day, whenever i can get all dirty while playing sports i'm happy. Just happens much less frequently these days than it did when i was growing up
contrary to the wild held belief of today, dirt and some germs are good for us. the sterilization of everything these days leads immune system to breakdown, little wonder our bodies are less able to cope.
Be sure to read the rest of the article; it is quite fascinating. It's also interesting to note that one of the related articles listed on the side is titled "Smarter People Are No Better Off"!