A terrorist plot named the 2006 Transatlantic Liquid Bomb Plot is why we have the 3.4-ounce limit in place for bringing liquids on airplanes. The plot involved a group of people carrying liquid explosives to assemble bombs in the air. Had they succeeded with the plan, at least 1,500 people would have died.

The 3.4-ounce (100ml) limit, known as the 3-1-1 rule, was implemented globally following the disruption of the 2006 transatlantic aircraft plot. Terrorists had planned to smuggle liquid explosives disguised as soft drinks onto multiple planes. Security experts determined that 100ml is the "critical mass" threshold; a single container of this size is generally not large enough to create a devastating explosion on its own, and the requirement to fit all containers into a one-quart clear plastic bag limits the total volume of liquid a passenger can carry. This restriction makes it much harder for a bad actor to combine multiple small containers into a larger, more dangerous device mid-flight without being noticed. While advanced CT scanners are beginning to phase this rule out at some airports, it remains the standard because it balances passenger convenience with a proven scientific safety margin.