The saying "What goes up must come down" is an appropriate starting point. If you fire a gun into the air, the bullet will travel up to a mile high (depending on the angle of the shot and the power of the gun). Once it reaches its apogee, the bullet will fall. Air resistance limits its speed, but bullets are designed to be fairly aerodynamic, so the speed is still quite lethal if the bullet happens to hit someone. In rural areas, the chance of hitting someone is remote because the number of people is low. In crowded cities, however, the probability rises dramatically, and people get killed quite often by stray bullets. Most major cities have laws in place to try to keep people from shooting guns into the air in celebration.
For a fuller explanation, read Fire a Bullet Straight Up: How High Does It Go?
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