So it's somewhere over here and that's what the red line is. Now this is for an ideal fluid. Now, in an ideal fluid the surface molecules are not connected by any intermolecular forces because remember, in an ideal fluid we're neglecting those intermolecular forces. So nothing holds the molecules together so they could freely escape into the environment if they have the kinetic energy. However, in a non ideal fluid there are intermolecular forces and these forces hold the molecules together. They inhibit the molecules from escaping into the environment.