We humans are naturally selfish creatures. Perhaps it's just me, but I think that the nature of this selfishness changes over time. As children, we are obsessed with owning or possessing things: our entire lives revolve around having toys, having food, having entertainment. But as we get older, we seem to become less concerned with having things, and more concerned with doing things. This strikes me as a fascinating change, and also as one of the key transitions between childhood and adulthood.