So, that will give me phi t, t plus g phi y is equal to 0 on y is equal to 0. So, this becomes the free surface boundary condition for whatever problems. This condition is a combination of the dynamic condition as well as the kinematic condition. Now, I will look at the whole thing. Then, what is y is equal to 0? y is equal to 0 is the mean x i. If I have mentioned, I call it mean free surface or my still water level, this is my mean free surface of the still water level.