A sand clock, commonly known as an hourglass, is a calibrated physical timepiece that uses sand falling from an upper container to a lower container to measure a predetermined amount of time. The archetypal hourglass is a glass vessel with an upper and lower bulb, joined by a narrow neck.
Factors that affect a sand clock include quality and volume of the sand, quality and size of the vessel's material and the tilt of the surface the clock is placed on. Any moisture can cause the sand to clump together, inhibiting its smooth flow and preventing accurate timing. For this reason, sand clocks are sealed after being filled.