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The law of gravity does not apply, until you look down.
Every nuclear warhead in the US arsenal is fitted with a limited-try Permissive Action Link device designed to prevent unauthorized or inadvertent use of the weapon. After three tries with the wrong code (out of 1 trillion combinations), the warhead will disable itself.
The PAL is the product of the nation's best and brightest engineers, and has been rigorously tested.
Unfortunately, the PAL can be disabled with a penknife, some Crazy Glue, and a stun gun.