Ben Light displayed Kinetic Sculpture 5 at the ITP Winter Show last month. When the user waves his/her hand over a photoresistor, a push solenoid is triggered, causing a rod to decouple and swing to the other side. Whatever losses are sustained due to conservation of energy are then recovered by a strong magnet that gives the rod that extra pull it needs to couple to an identical solenoid on the opposite end.
A row of five rods lets the user play with sequencing the movement, which alludes to classic demonstrations of harmonic motion. A maker with a nuttier sensibility would think this project is on track to be a perpetual motion machine, but I think Ben knows better.