One of my hobbies is woodworking. I needed a few shelves in the shop that would be carrying some heavy loads. So, I made these T-boxes. Each is 4' long, 15" deep, and 1-3/8" thick. Take a look a the "sag":

That's me standing in the center, about 200 lb. The shelf is supported only at the very ends by a couple of benches I made. I used the chair to help get up there, so it's a foot behind the shelf. The steel tube is resting as an indicator. This shelf is mostly hollow and weighs less than 2 lb. Kind of defies gravity, doesn't it? Wonder what Sir Isaac Newton would say about it?