Give it a push and Santa rocks side to side, skis parallel, arms swaying, the snowflake counterweight pulling him back into rhythm each time. This is the rokr gravity swing mcd06 — a three-part wooden balance toy that demonstrates potential and kinetic energy through a pendulum system with no motor, no batteries, and no external input once it's in motion. About three hours to assemble, 150-plus laser-cut wood pieces, and a finished model that keeps moving long after you set it down.