Trestle Track | Builder Set 8+Made up of hollow stacking cubes and a variety of perfectly flat tracks. The paths cut out of each track widen almost imperceptibly from one end to the other, creating an invisible slope that propels the marbles forward. Watching them go is like magic in slow mo! Medium Set with 73 pcs Unique marble run with marbles propelled by widening tracks Encourages spatial reasoning, creativity, engineering skills, problem solving Tracks suspended by hollow