The values of the contact angles of different liquids on the same solid are classically interpreted by theories assuming that the vapor adsorption is negligible on solids presenting contact angles. This implies that the solid may be defined by a certain tension, called "critical tension", which is not clearly thermodynamically defined. These assumptions are in disagreement with modern theories of vapor adsorption science. In this paper, it is shown that it is possible to understand the contact angle values using a rigorous derivation of the Young equation, which links the contact angle, the liquid-vapor surface tension of the drop, and the surface tension of the solid in equilibrium with its own vapor. Numerical approximations allow the contact angle variation to be predicted, without use of ad hoc definitions. It is then possible to link vapor adsorption results and contact angle experiments. It is also possible to deduce the surface tension of solids simply and to compute contact angles on powders without using the Washburn method.