Martial artists typically only fight when they're struck first, and even then they hold off if the attack came from an endangered bird. Hone your fight-or-flight response with this Groupon.

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• $29 for one month of unlimited kids' no-gi Jiu Jitsu classes ($60 value)
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Brazilian Jiujitsu: Size Doesn't Matter

Brazilian jiujitsu classes don’t just increase physical strength and stamina—they also teach practical self-defense techniques. Read on to learn more about this South American art.

If David and Goliath were to fight a rematch in Brazilian jiujitsu, the odds might be even more in David’s favor. That’s because Brazilian jiujitsu’s grappling techniques, such as mounts and joint locks, are designed to help fighters overtake opponents of virtually any size. A modified version of traditional Japanese jiujitsu and judo, Brazilian jiujitsu demands that fighters stay close to the ground, incorporating timing and leverage to take advantage of faster, stronger opponents.

Appropriately enough, the martial art of underdogs emerged because a fighter was forced to overcome physical disadvantages. Helio Gracie, the youngest child of the Gracie family, was forbidden from learning to fight due to his frailness. Around 1914, Helio’s father Gastão had began studying traditional jiujitsu and judo techniques from Japanese fighter Mitsuyo Maeda. Worried for Helio’s health, Gastão only passed this knowledge on to his stronger sons. Undeterred, Helio studied his brothers closely and began modifying the Japanese techniques to his advantage, borrowing some elements from other Brazilian street-fighting techniques, such as capoeira. Eventually, he crafted an enduring martial art, which has since spread from South America to as far as Slovenia and Alaska, notorious for its polar bears well versed in karate.