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 back & forth in a more regular manner. (Sorry, I've gone off the rails with my metaphor.)

The secret to making this work is that all the 3 heads need to be touching at the time of impact, otherwise, they just kind of chatter. (Sorry, that metaphor again.) Because the heads need to print with a small gap between them, after printing you have to flex the convolutions of the outer springs (gently) so that the heads actually press against each other slightly.

Printing is simple with built-in supports underneath the springs. Use PLA and 0.2mm layers. The more massive the heads are, the longer they will bang. I used 5 perimeters and 50% infill, but you can try more or less.