A glass hourglass with falling sand grains that pile up below, then resets in an endless loop. In real granular physics, flow rate through a narrow orifice follows the Beverloo equation and is famously almost independent of the sand height above the neck — unlike a liquid, whose flow slows as the reservoir drains. This happens because grains form self-supporting arches near the opening that briefly jam and release, regulating a steady trickle. Traditional hourglasses were precision-calibrated to run exactly one hour (or 30 minutes for maritime "watch glasses"), and ships once relied on stacks of these glasses tied to bell-ringing schedules to track shipboard time before accurate clocks existed at sea. Drag to orbit, scroll to zoom.