The zeroth law is so obvious that it wasn’t added as a law until after laws one through three were developed. It is based on thermal equilibrium between two bodies. As has been stated, if two objects have different temperatures, heat will flow from the hotter to the colder. If there is no temperature difference, there is no net heat flow. They will be in thermal equilibrium. The zeroth law states that if objects A and B are in equilibrium and B and C are in equilibrium, then A and C are also in equilibrium. Suppose object B is a thermometer. You put it in contact with object A. Heat flows until they are at the same temperature. You then move the temperature to object C. If the thermometer shows no change, then B and C are in equilibrium and we can conclude that objects A and C are at the same temperature.