Everyone's first lesson in stand up, somewhere in the first few minutes, the trainer tells you to keep your hands up. The lesson is repeated at some point in every stand up class ever.
The reason it has to be repeated so much is that during a fight, it is hard to remember. If you forget the lesson, if when your undefeated professional opponent goes southpaw, reversing the expected arc of every strike, instead of buttoning up, you lower your guard and throw a tentative jab, well, what can happen is... this.
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