stuff that moves in an hourglass

In an hourglass, the material that moves is typically a granular substance, most commonly sand or similar particulate matter. This material flows from the upper bulb to the lower bulb through a narrow neck due to gravity. The flow is regulated and continuous, allowing the hourglass to measure a specific passage of time. The particles move in a pattern where they form a flowing vortex as they pour through the neck and then pile up in the lower bulb. The granular material can be sand, powdered marble, tin/lead oxides, pulverized burnt eggshell, or other fine powders chosen for their consistent flow properties. The size of the granules relative to the neck width is important to maintain a steady flow without clogging. Thus, what moves in an hourglass is a stream of granular particles, historically sand but often other fine particulate substances, flowing under gravity through a narrow passage from one chamber to another.