And convection absolutely requires the presence of molecules or atoms within the medium. There is no longer a need for a physical presence or a physical bridge connecting two objects or two systems, but there is a need for those molecules, okay? And here we have a common example used to describe or convey convection, okay? Here we have a cup of coffee. You know that if you leave a cup of coffee, as in temperature, eventually it will cool down. That's because energy transferred from the cup of coffee to the air, okay?