Liquids rules exist because of one specific 2006 plot

What follows is the working version of the liquids rule: the decisions in the order you actually meet them, with the reasoning attached.

The origin is specific

Restrictions on liquids in cabin baggage were introduced across most of the world in August 2006, following a plot to use liquid explosives on transatlantic flights. The initial response banned liquids almost entirely, and the 100ml-in-a-transparent-bag compromise emerged over the following months. The volume threshold reflects the quantity assessed as necessary to cause structural damage, not a round number chosen for convenience.

The transparent bag is part of the same control rather than a tidiness rule: it caps the total a single passenger can carry and forces every container into one view for the screener.

Why it lasted so long

Conventional X-ray screening produces a two-dimensional image and cannot reliably determine what a liquid is. Without that capability, the only workable control was limiting quantity, which is why the rule survived nearly two decades.

Screening technology, not threat assessment, was the binding constraint. Machines that analyse a single sealed bottle do exist and are used for exemptions such as medication, and they are far too slow to put every passenger through, which is why they never replaced the limit.

CT scanners change the calculation

Computed tomography scanners build a three-dimensional image and can characterise materials, including distinguishing benign liquids from threats. Airports installing them can relax both the liquids rule and the requirement to remove electronics from bags.

Rollout is expensive and slow, which is why the transition has been staggered and repeatedly delayed. The units are also considerably heavier and larger than the X-ray machines they replace, so some terminals need structural work to the screening hall before one can be installed at all.

The transition is the confusing part

Rules now vary by airport rather than by country, and sometimes by terminal within an airport. An airport that has relaxed the rule on departure does not help you at a connecting airport that has not. The safe approach during the transition is to pack to the strictest rule on your itinerary.

Some airports that had lifted the limit have since reimposed it, temporarily or otherwise, so what you experienced there last year is not evidence about this month.

Practical packing regardless

Solid alternatives — bar soap, solid shampoo, toothpaste tablets — remove the problem entirely and take less space. Medication and baby food are exempt in most jurisdictions but generally need to be declared and sometimes documented. Duty-free liquids bought airside are sealed in tamper-evident bags, which are valid only if unopened and sometimes only within a time limit.

Nine times in ten, that catches people transferring through a third country, where the purchase can be taken at the next security check unless the receipt is still sealed inside the bag with it.

Check the current rules for your route before you rely on any of this.

What still triggers a bag search

Dense organic material, tangled cables and stacked electronics produce images screeners cannot resolve, and a search follows. Packing electronics in a flat layer and keeping cables in one pouch measurably reduces the chance of being pulled aside.

The working rule: this applies even where devices no longer have to be removed, because the scanner still has to see through the bag. Food is the other reliable trigger, since cheese, chocolate and dense preserves present much like the materials screeners are trained to stop on.

The takeaway

Pack to the strictest airport on your route, and switch to solids where you can.

The trip goes better when the bag is boring.