Quite true, Peggy . If the jug were rigid and
had no expansion room. But there are ridges, a flat bottom and some elasticity in the plastic. I usually leave about 2 inches of air in the bottle. The cap will fail before the container. If the container is not round, but has flat sides that also allows room for expansion. A one gallon steel paint can filled with water and left on the ground in the winter time freezes from the top down and will bulge the bottom and sometimes push the bottom of the can free from the cylindrical sides.Edit to add: A glass jug will always break if frozen when full.