You just got done with your lunch and you are just finishing up your grapefruit. You get full so you throw it on the ground thinking the birds would get it. It has been a few days and the grapefruit is still there. This grapefriut is now a closed system, it is isolated and doing nothing. A few weeks pass and the grapefruit is now decomposing and turnig into dirt because of thermal energy. Not all of the grapefruit is turning into dirt some of grapefruit is turning into a gas. The grapefruit is now dirt ans gas but the mass of the object is still the same, because the Law of Conservation of Energy states matter can't be created of destroyed, it just changed state.