Never evaluate someone according to his/her size, stature, age, education etc and never allows anyone to know your, weakness and weapon, otherwise you will be defeat. Goliath failed because he looked down on David according to his size and age coupled with the stature. He thought that only what David has is the sling and the stone not know that David kept his weapon away from him and showed him the physical weapon that has no power but was powered by the unseen power rested on David, that later got Goliath down.