The hat size you wear determines how big your brain is.
If your a "One size fits all" kind of head, your head is so big you can't fit XL sizes so your smart because you have a big brain. And if your a small sized head, well... your not as smart as the XL heads.
:P
EDIT: New rule, only choose "One size fits all" if your head is bigger than XL, ok? :P
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I got a big fat head. According to this hat its an L/XL and it doesn't fit me.