We all know that because if y zeta is the air surface and this the water surface. For all time when space very good d by d t y minus eta x t 0 that means there is no gap. This is there is no gap between the air water. That means this is on y is equal to eta x t on the free surface. Total derivative is 0. That is there is no air and water surface. If I simplify this equation; it is because d by d t is nothing but the material derivative. (Refer Slide Time: 15:37)