When it comes to judging how much weight a piece of furniture, a shelf, or a stand will hold, each of us has a calibrated weight to test it with - our own weight. We can lean on the top, swing back a forth a little, climb on top, even do a deep knee bend. For things we want to hold a much heavier weight we can enlist another calibrated weight to help us. There is a risk of injury doing this, but instinctively we should only gradually build up how much weight we test it with. Isn't if far better to use that testing method than asking us to make our educated guesses about it? 400 pounds of water and 400 pounds of friends both apply the same load!