Thanks - my limiting factor here was the small 'stone that I used. I won't have time to build a larger one this year (too many other projects) so I had to come up with something that would give me the extension I needed without sticking out too far behind the 'stone. The retracted height of the mech is ~20" and 40" extended. The pivot that the head is attached to is actuated using a length of piano wire and a spring. The head hangs well out of view when down. Lighting will help - a small flood in front of the 'stone will black out the background, and a UV light will be used to draw attention to the skull as it swings up. An MP3 player will provide a loud scream.
Maybe we're more than what we think we are...
...or maybe not.