This rule came into play after a plot was uncovered in 2006 to bring down airplanes using liquid explosives. To make it harder for bad guys to carry out such plans, authorities limited the amount of liquids you can bring in your carry-on. Each passenger is allowed to bring liquids in containers of no more than 3.4 ounces, all fitting into a single quart-sized bag. It's all about keeping everyone safe up in the air.