The Golden Cylinder

You are shown two cylinders - one of them is made of gold and hollow, the other one is made of alloy and solid. Both of them have the same weight, shape, and are colored in black. How can you figure out which cylinder is the golden one, without scratching the paint?


Simply roll the the two cylinders on the ground. The hollow one will roll farther than the solid one, since it will have higher moment of intertia.