The frog was living dangerously until physics stepped in! That equation μN ≥ mg is basically saying "the force of friction better be greater than or equal to your weight, buddy, or you're going DOWN." Static friction (that clingy feeling between surfaces) is what keeps the frog from sliding down the wall. Without it, gravity would turn our amphibian friend into a frog pancake. Next time you're defying gravity on a vertical surface, thank the coefficient of friction for not letting you become a tragic physics demonstration!