The hourglass is a simple device, many centuries old, used to measure short intervals of time. Sand, or some similar material, is allowed to drop from one glass vessel to another. These vessels are connected by a thin neck. The device is calibrated so that the sand passes into the lower vessel in a certain period of time. This Demonstration simulates how an hourglass works for a selected time period, as the sand grains drop to form a pile with an angle of repose of 34ÃÂº.