Why is there a 100ml liquid rule for hand luggage?
It dates from a specific event. In August 2006, UK police disrupted a plot to detonate liquid explosives carried aboard transatlantic flights in soft drink bottles. Restrictions were introduced within days, and the 100ml limit became the international standard shortly afterwards.
The logic is quantity rather than detection. Screening equipment at the time could not reliably distinguish a benign liquid from a dangerous one, so instead of identifying threats the rule limited the volume any one person could carry through. The associated requirement — containers in a single transparent resealable bag of about a litre — caps the total as well as each item.
The rule covers anything that can flow or be squeezed: drinks, gels, pastes, creams, aerosols. It applies to the container size, not how full it is, which is why a half-empty 200ml bottle is refused. That surprises people every day at every airport.
There are established exemptions, generally including baby food and milk, prescription medicines, and special dietary requirements, though these usually require declaration and may involve additional screening.
The rule is being phased out in places. Newer computed tomography scanners build a three-dimensional image and can identify materials well enough to assess liquids without removing them, and airports fitted with them have relaxed or removed the limit. Rollout has been uneven and several deadlines have slipped, and some airports that relaxed the rule later reinstated it.
That unevenness is the practical trap: the airport you depart from and the one you connect through may have different rules on the same journey.
Check the specific airports on your route before travelling rather than assuming the rule you last encountered still applies.