Fig. 7.2.2 The movement of sand through an hourglass.When you examine the flow of sand through the hourglass, the flow is steady and at an
even pace. All things considered, the flow of sand is done efficiently.
So, how is the flow through the hourglass done in such an efficient manner? Consider the
grains of sand as they pass through the center of the hourglass. Fig. 7.2.3 shows the flow
of sand through the center of the hourglass.
Chapter 7.2: The Standard Work Unit