My son and I were playing in the park and accidently came across an interesting physical force. We threw a marble on an angle through the inside of a plastic blow-mold cylinder. The cylinder was approx 1M long X 0.6M Diameter. The marble returned after reaching top dead center on the second loop. If thrown hard enough it would spiral from one end to the other until it dropped. We continued the excercise from each end at different speeds with the same result so long as the marble never left the surface of the cylinder. What causes the marble to return and how? Any ideas? Thanks!