The number of people harmed by a recent tornado in the U.S. state of Oklahoma was greatly reduced, thanks to a special early-warning system.

On Monday, a major tornado hit Moore, a suburb of Oklahoma City.

A tornado is a violent storm that usually looks like a whirling funnel.

Residents were warned there was a big storm coming, and possibly a tornado, days before it arrived.

When it actually hit, sirens blared, giving people a 16-minute head start to find shelter.

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