So, then this condition, this is on y is equal to eta x t. Then, this condition will give me del phi by del y is equal to del eta by del t plus phi x del eta by del x. This is satisfied on y is equal to eta x t because this condition I call this is my kinematic condition on this condition is satisfied on the free surface. So, there are two conditions, which are satisfied on free surface. One is the dynamic condition, another is the kinematic condition.