Floating Hourglass Puzzle: famous physics puzzle- why does the hourglass not start rising right away? The glass only ascends after about half of the sand has transferred to the lower bulb. The hourglass is a normal glass timer, it is submersed in plain water, and the main tube has an inner diameter which is at least 2mm larger than the diameter of the glass. A simple physics property of the hourglass changes and explains this behavior. I'll post the answer in a day or so. Follow this link for in