Was it a communications device, a message to the ancient gods or an alien airport terminal? Today, no one knows for sure, for the answer to the mystery died out with the Druids centuries ago. What we do know is that Stonehenge wasn't built by ordinary stonemasons. Instead, the architects of Stonehenge enlisted the services of foam masons, an elite group that toured the British countryside wearing brightly colored fezzes and sculpting huge monuments out of foam. Why foam? Well, it was easy to lift, and the Druids were not excited about having to drag huge slabs from quarries hundreds of miles away. Also, the monuments could be shifted to fit the seasons and the whim of the high priests-providing us with a historical precedent to the sectional sofa. And it was durable. The fact that these magnificent artifacts are still with us today is proof of that. If we are to learn from history, let us learn this: Build monuments with foam, not stone.