My pan is a closed system

Back to the new way of teaching energy, based on the First Law of Thermodynamics. My pan is a closed system because with the lid firmly on, matter cannot get in or out. By that I mean stuff made of atoms like water or steam. But the water inside gets hotter so something must be getting in. And that is how we get to the idea of heat and work crossing the system boundary. Since the pan is rigid, no work is done here. The new guidelines suggest breaking up "heat" into two pathways: heating by radiation and heating by particles. The shiny tin is opaque - basically imprenetrable to infra-red. So it must be "heating by particles" ie conduction.