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.
When humans discovered fire, they learned to cook their food in a controlled environment. What came next? Flamethrowers, of course. What do flamethrowers have in common with their living counterpart -- fire breathers?
The Boeing B-29 Superfortress was the biggest, most expensive gamble by the United States during World War II, built with incredibly high expectations on a tight deadline. Read about the big advances in aviation technology brought about by the B-29.