In chemistry, a steady state is a state in which all state variables are constant in spite of ongoing processes that strive to change them. For an entire system to be at steady state, i.e. for all state variables of a system to be constant, there must be a flow by the system (compare mass balance). One of the easiest examples of such a system is the case of a bathtub with the tap open but without the bottom plug: after a certain time the water flows in and out at the similar rate, so the water level (the state variable being Volume) stabilizes and the system is at steady state.