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So what exactly are the reasons:

  • policy by the airlines?
  • slowing down security?
  • What if it's 100mL of water filled in one of those tiny travel bottles that people use to downsize their toiletries?

I was just discussing this topic with a friend & then realized while I know of the rule I would like to know all the aspects as to why.

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    en.wikipedia.org/wiki/2006_transatlantic_aircraft_plot it's a security theater, basically
    – JonathanReez ♦
    Commented Feb 17, 2015 at 9:52
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    AFAIK you can take a bottle of water on the plane, just not through security. You could for instance take an empty bottle through security and then fill it up at the gate.
    – drat
    Commented Feb 17, 2015 at 9:56
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    @Jonas: In various places, such "bottle-filling stations" are commonly known as "taps" :)
    – O. R. Mapper
    Commented Feb 17, 2015 at 14:10
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    Just as a note, while the comments so far are correct that you can usually buy or fill a water bottle in the terminal and take it on with you, this is not always the case. In particular, some international flights actually confiscate water bottles at the gate or even inside the boarding bridge. See this question and this question.
    – reirab
    Commented Feb 17, 2015 at 16:25
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    I once read (source: the internet!) about a guy who tried to bring a bottle frozen water through security, claiming that, hell, "it's not liquid!"!
    – Pierre Arlaud
    Commented Feb 18, 2015 at 10:28

4 Answers 4

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Because it's difficult to tell apart a bottle full water from a bottle full of a chemical like hydrogen peroxide that could be used to make liquid explosives. There was one hare-brained terrorist plot that apparently tried this in 2006, and because "passenger convenience" will always lose out to "bureaucratic ass-covering" when it comes to security theater, all liquids of all kinds were banned by the TSA. (Unless they're under 100ml, so yes, you can take a 100ml bottle of water on board... if you can find one!)

In Japan, they've already got bottle scanners that can identify suspect liquids. These are increasingly being adopted by other countries, and once they're widespread enough, the liquid silliness will hopefully end.

Note that while you can't take a partly or completely filled >100ml water bottle through security, it's perfectly fine to take an empty bottle through security, and fill it up before you board the plane. Or you can buy a bottle once you've passed security.

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    Are 100ml of hydrogen peroxide not sufficient to make liquid explosive? Or is it just that the explosion might not kill everyone on the plane? Ah you gotta love security theatre.
    – JoErNanO ♦
    Commented Feb 17, 2015 at 11:16
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    @JonStory For maximum security, you could live in a bunker and never fly! Discussions on if the current rules are actually in any way sensible or proportional probably belong on Skeptics.SE though
    – Gagravarr
    Commented Feb 17, 2015 at 12:37
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    I like how, having confiscated what could be a bottle full of explosive, they (often) throw it into a box containing a lot of other previously confiscated potential explosives.
    – bye
    Commented Feb 17, 2015 at 17:11
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    This is getting a bit off-topic. Perhaps someone should post the question, "Ways of blowing up a plane with at most 100ml of liquid".
    – Cary Swoveland
    Commented Feb 17, 2015 at 23:36
  • Comments are not for extended discussion; this conversation has been moved to chat.
    – RoflcoptrException
    Commented Oct 25, 2016 at 20:22
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tl;dr: Airports like to sell drinks (and it may increase security)

As described by @jpatokal it is indeed possible to make explosives out of some liquids, and thus airlines used security as a reason to ban all liquids.

Though the amount of terrorist attacks prevented by this is probably in the range of 0-1 (which could of course make it worth the effort), there is a side effect that is easily noticeable:

Since it is harder for people to arrange their own drinks, the number of drinks purchased at an airport increases. I have been unable to find a citation for this but can attest to this from personal experience.

So, preventing people from taking bottles may increase security, but it will definitely increase revenue. And this is at least part of the reason why the rule got enough support to be implemented.

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    This is skeptical, but anyhow the revenue increase is a positive side effect and wasn't the original plan.
    – Nean Der Thal
    Commented Feb 17, 2015 at 13:47
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    In general we can of course never know why certain things are done. But the economical benefits here are clear, and those tend to result in support. Therefore I think the economical angle should not be overlooked when someone asks about all aspects leading to the current situation.
    – Dennis
    Commented Feb 17, 2015 at 14:13
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    What is really annoying is that, even though once you are past the check in stuff is tax-free, a bottle of water will cost you more than double the "external" price...
    – algiogia
    Commented Feb 17, 2015 at 15:21
  • You should put some emphasis (italics) on "may" in the bold header.
    – Cole Tobin
    Commented Feb 17, 2015 at 17:31
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    @MeNoTalk did you mean "cynical", rather than "skeptical"?
    – Golden Cuy
    Commented Feb 18, 2015 at 0:10
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The policy is actually fairly reasonable, and the base reason is that they can't run an analytical chemistry laboratory at the checkpoints (nor find staff that can both understand the results and work for a government salary).

The hydrogen peroxide mentioned in another answer is one possibility. Looks just like water. The stuff you buy at the drug store makes a great antiseptic or toothpaste, stronger mixes make good rocket fuel. Gasoline looks a lot like apple juice - light up 500ml of Regular Unleaded and ask yourself if you want that happening in the window seat. 100ml bottles of apple juice are rather rare and will attract attention. Other chemicals not mentioned here can start roaring blazes on contact with air - you just have to open the cap.

Sales were certainly not a consideration - every airport I've been to charged market rate for drinks after security, one openly advertises (since the 1990s) that prices after security are exactly the same. And I've had no problems at all bringing a freshly rinsed thermos through. It gets a quick glance to see if its empty.

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    If your explanation should hold, there has to be a complete ban on bringing liquids through security. If five terrorist buddies are allowed to bring 500ml of unleaded petrol through security and it is a real problem, we have an immense security risk.
    – Tor-Einar Jarnbjo
    Commented Feb 17, 2015 at 18:05
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    @user568458 - that's exactly what happens if you bring through water in a baby's bottle - one of the few exceptions to the liquid rule.
    – paj28
    Commented Feb 18, 2015 at 11:01
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    @paj28: best comment ever on this topic: "fresh milk for infants: no limit when transported in the original containers"
    – paul
    Commented Feb 18, 2015 at 12:57
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    If this was really about terrorism, then there would be just a limit on the total amount of liquid you are allowed to bring. The wired rule that liquids have to be in max 100 ml containers (even though you are allowed several of them, so if you split the forbidden 500 ml bottle among 5 smaller bottles its suddenly fine), leads to the conclusion that the official reasons are bullshit, and its just about increasing revenue from overpriced drinks. A dangerous 500 ml doesn't suddenly become safe by splitting among 5 bottles, Especially that you can combine it back once on the plane.
    – lowtoxin
    Commented Jul 28, 2016 at 1:59
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    "every airport I've been to charged market rate for drinks after security" Haha. Hahahahaha. Hahaha. Ha. Nice joke. I mean, er, wow, you go to airports that are friendly to their customers. Every airport I've been to charges a noticeable mark-up. Some charge a ludicrous mark-up: I paid EUR3.50 for a 750ml bottle of water at Lyon acouple of years ago, for example (it might even have been 500ml).
    – David Richerby
    Commented Sep 22, 2018 at 18:13
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If you ask them, it's to prevent terrorist attacks in the form of bombings. Apparently several people posting agree. I would answer that it is to appear to prevent terrorist attacks. I say that because tsa is slightly more effective than mall cops.
Could there be bombs created through that nefarious methods that get caught by the liquid limitations? Maybe. Probably not. TSA got caught missing something like 95% of things getting past them by homeland security. So hope it's that 5%. It isn't like multiple people couldn't pool together small bottles of liquid.
The purpose is to appear busy and to make people feel safe by being very intrusive. The reason we haven't seen more attacks is because of increased intelligence operations, and because we reinforced the cockpit door. Without the ability crash the plane into something, we're back to the pre-911 days, where they can attack the plane, but it's not nearly as good a target.

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  • The reinforced cockpit door was the cause of one of the worst attacks of 2015, that security measure clearly backfired. And increased intelligence hasn't helped much. Terrorists still fly (and many that aren't are incorrectly banned). What has helped to some extend is increased security. Many reports show prevented attacks and captured terrorists at the gates. But as the 'underwear bomber' showed, the intrusive checks in the airport are not foolproof yet.
    – Abel
    Commented Jan 6, 2017 at 21:06
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    Why do people always say "you can just get 5 people to get together and..." - when even trying to make software in teams like that leads to squabbling and fighting. The more people need to team up to attack, the more likely the plot will collapse or be discovered in advance.
    – Kate Gregory
    Commented Jul 2, 2017 at 13:14
  • @KateGregory Well, the original plot was to blow up six planes, so that involves more than five people. But you only need one person to carry 500 mL, you just need 5 containers.
    – jinawee
    Commented Apr 25 at 12:18

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