Giant Egg Warmer
In the 1970s the US army did an experiment where they dressed soldiers in artic weather combat gear and sent them out in the snow. They then asked them to remove their hats and stand in the -30c storm force winds. The soldiers reported having cold heads. This scientific experiment led to the US army declaring that humans lose most heat from their heads. Since then hat sales have boomed and soldiers no longer wear trousers.
One size fits most heads.