The reason liquids can't be brought on planes in large quantities is primarily due to security concerns. This regulation was implemented after a 2006 foiled terrorist plot (known as the "liquid bomb plot") where al-Qaeda operatives planned to detonate bombs made from liquid explosives on planes. To prevent such threats, aviation authorities limit the amount and packaging of liquids passengers can carry onboard to reduce the risk of liquid explosives being brought onto the aircraft.

The typical rule is that liquids must be in containers of 100 milliliters (3.4 ounces) or less, all fitting within a single transparent plastic bag with a maximum total volume of about 1 liter. This "3-1-1 rule" helps security personnel effectively screen liquids without allowing enough volume to cause harm. Exceptions exist for essential liquids like medications, baby formula, and certain duty-free liquids, but these often require additional screening.

Liquids that can be a threat include not only water and drinks but also gels, creams, aerosols, and pastes, such as toothpaste, shampoo, and lotions. Completely frozen liquids are often allowed if they are solid, but partially melted ones are treated as liquids and subject to restrictions. The primary goal is to prevent any liquid substance from being used to assemble an explosive on the plane, ensuring traveler safety.

In summary, liquids are restricted on planes as a crucial security measure to prevent terrorist attacks involving liquid explosives by limiting the quantity and type of liquids passengers can bring onboard.