Head Bangers

These guys are always butting heads, especially when debating Newtonian physics. They're an inverted version of the classic Newton's Cradle demonstration of conservation of momentum and energy. But why is the middle spring so much stiffer? Because of imperfections in the collisions (energy loss, misalignments, etc.), the motion can devolve into a 'sway' mode, where all the heads sway together. With the center spring stiffer, the middle guy acts as a steadying moderator, keeping the debate going