From a post on L19 by BaghwanB:
Elevator-pitch answer to "How do you play this?". Goes like this:
This is an empty field and we are both running fence to see who can get the most. The stones are the fenceposts and you fill in the fence when you get pushed. To keep it interesting (and not turn into "My side, your side") you can pull out the posts if you surround them yourself.
Kinda rural-based, but seems to get the idea across to most people quickly. Feel free to use at your leisure. Any talk of "eyes" or "living groups" comes later.