skin depth

http://en.wikipedia.org/wiki/Skin_effect

Hi:

consider a waveguid which is working in the daminant mode . why the wave propagate in the length of waveguide and not propagate across the waveguide ?
now you consider a microwave oven . why the food cook in several minuit ?
in the former case the skin depth of the waveguide is very low and therefor the wave can not penetrate in it then propagate only in the length of it . in the latter case the skin depth is too long and the wave can penetrate easily and deliver the energy to food.