In my younger days we used to boil water in paper and styrofoam cups in camp fires.It was a fun trick to show city kids.The cups would burn or melt down to the level of the water in it.We would then burn an empty beer can like it was paper in the coals.I'm not sure of the physics involved but it worked.I think that in Craig's situation the aluminum foil burned because the water in the container couldn't cool the outer layers of foil.