The thickness of the foil is very important. It is meant to flaxiltate the shape shifters ability to identify human presence. It has nothing to do with blocking. Kind of like a metal detector picking up a foil signal when it is actually a chunk of gold. The interpolating effect of Flaxiltation is critical. It can be the difference between these garter wearing feline shape shifting vampires sinking their fangs into you or you remaining invisible to them.
Unless they are accompanied by the body heat sensing flying monkeys.
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