"Imagine the earth as an apple. Cut it into four pieces. Three of those four represent water. "
This is a lovely flash animation the looks at how much of the earth is usable for agriculture. I wish I could embed this, but you have to go here to watch it. It really makes the point that there isn't much of the planet suitable for growing food--and we're covering a lot of that with cities. Because, for some reason, we seem to think that building cities and paving over the best,most productive,farm land on the planet is a good idea. Go figure.