An interesting variation on the Pitch Drop Experiment http://www.physics.uq.edu.au/physics_museum/pitchdrop.shtml might be a sealed barrel or jug half-full of pitch rolling down a gentle incline. I'm trying a similar experiment with bottles of "gel-caps" (or "soft-gels") of oil-soluble vitamins (e.g., E, A, or lycopene) which adhere when left undisturbed, but gradually separate under very light force, making little clicks and rattles minutes after their bottle is tilted. The problem is to get the capsules detaching one or two at a time--too steep an incline "liquefies" the whole inventory and the bottle accelerates. Likewise too full or too empty a bottle, so a math-fun tie-in is to find, given the bottle weight and density of the "fluid", the fill depth which minimizes the height of the center of gravity. --rwg