[Warning: bad joke ahead]
Every day at noon a soldier fired a cannon to signal it was noon. A guy was curious as to how he knew when to fire the cannon. So he asked the soldier, who told him "the guy in the guard station gives me a signal, and I fire the canon". He asks the guy in the guard station how he knows when to signal, "I use the clock on the wall, a guy comes and sets it occasionally". He finds the guy who sets the clock and asks him how he knows what time it is, "I sync my watch to the clock in the town square, then set that clock from my watch". So he finds the guy who sets the town square clock and asks how he knows what time to set it to. "Oh, I just sync it to the noon cannon".
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