Textures on the two ends of the glass, and particles to show
sand flow. A mesh-sand 3D hourglass would be a nice feat to
accomplish. That's tricky. The shaders on the ends might need a
textureModeList = #reflection, so they keep right-side-up when the
hourglass turns... but then it moves as the camera turns, too, so
probably no good. You'd have to rotate and place the shader
correctly according to the rotation of the glass.