The second way by which energy is transferred due to heat is called convection. Convection is a transfer of energy due to the movement of fluids. And fluids can both include gas and liquid, okay? 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?