I watch the Astral Tours guide giving a demo of dowsing at Avebury.
He stands between the Devils chair and stone 98 and demonstrates that the rods cross if you walk right and open if you walk left.
If I'm feeling impish I point out to the crowd that there used to be a Toll house at the begining of the Kennet avenue and the water pipeline is 4ft underground and runs between the 2 stones :)

(Astral Tours don't like me very much ;)
PeteG