This is a post by Cat McClintock on the PTC site.
What if all CAD users could see the un-seeable in their designs? For example, air, the inside of a bubble, heat, nitrogen, oxygen, pressure, pressure forces, or even water (because it’s usually in a pipe or in a pump or under the earth or seeping into your house).
Let’s call these un-seeables in your CAD designs and models, digital fluids. If you could model these digital fluids in your CAD system, you could save your company dollars in hardware prototyping costs and offer your customers lower cost products with the money you save.