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Yes, this is similar to another question, but, judging from the answers, maybe the OP wasn't clear enough in their question. According to the TSA's 3-1-1 rule, you can bring (in your carry-on baggage) as many 100ml containers of liquid that you can fit into a 1-quart (20mmX20mm) bag.

My confusion is this: If I can put the same liquid in each 100ml container, then the 100ml isn't a limit in any meaningful way; the 1-quart bag is. What is the rationale for letting me bring a bag of, say, four 100ml bottles of something but not one 400ml bottle?

The answer cannot be "Because 100ml of some explosive/corrosive liquid is not enough to harm the aircraft" because, from above, you can bring aboard probably 400-500ml of it in separate 100ml bottles in the same 1-quart bag. It can't be "Because, separately, they don't pose a threat" because one could mix them in any of the large empty containers they can bring through. The answer also can't be "Because TSA needs concrete, simple rules", since it would be simpler to just say "Whatever liquids you can fit into a 1-quart baggie" (i.e. a "1-1" rule instead of a "3-1-1" rule).

Seriously, what's the point?

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    Who said there had to be a point? – Michael Hampton Aug 1 '19 at 1:32
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    This is a rant disguised as a question, and responses beyond what was offered in the cited question will be primarily opinion-based. I voted to close. – DavidSupportsMonica Aug 1 '19 at 2:03
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    It's even weirder. Individuals can take up to 10 100ml containers, but obviously not a 1L bottle. For 'safety'. However a dodgy person can take an empty 1L container, which the others could be combined into once onboard? (facepalm) – Mark Mayo Aug 1 '19 at 2:37
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    This seems a lot like security theater, as Bruce Schneier has explained in his essay. A lot of time, adding more restrictions can make some people feel safer, even if it isn't actually making them safer. The TSA could use the excuse that combining the bottles would be suspicious (although you could do that in e.g. a bathroom in the airport), though I can't think of any practical reason. – Gyðja Björnsdóttir Aug 1 '19 at 2:48
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    Well, the video presents the most-plausible explanation I've heard: basically, that of the chemical compounds potent enough to present a threat at quantities below 1000ml, almost all would self-detonate (in a less-destructive way, presumably) if a terrorist were to try to combine separate bottles in the lavatory in the terminal or aircraft. Googling a bit on one of the chemicals in the terror plot which led to the liquid ban reveals that it can be quite problematic when exposed to air. So, now it's starting to make a little sense. – Jemenake Aug 1 '19 at 4:19
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Why am I allowed to pass airport security with 3 bottles of 100ml liquid but not with one bottle of 300ml? answers your question.

Rahul Dobriyal, pilot aspirant. Answered June 23, 2017.

It is not about the quantity of liquid you carry. It's about the size of the bottle in which you carry it. I would explain it by an example :

Let us consider that a group of terrorists plan to detonate a liquid bomb in the aircraft. Since a liquid bomb is very unstable in final form, they would plan to mix the liquid (say A) with another liquid or reagents(say B) on board to make the liquid bomb.

You are allowed to carry liquids in transparent 100 ml bottles. The quantity is restricted to no of bottles that can fill up a 1 Litre zipper bag. Now even if they carry liquid A in 10 bottles and B in another 10 bottles of 100 ml, it would be very difficult to mix the liquids together in precise amounts to make the bomb on board.

So, it is not about the quantity of liquid you carry. It's about the size of the bottles in which you carry it.

Edit 1 : In addition, it is easier to scan what kind of liquid you are carrying in 100 ml bottles. In bigger bottles you can disguise the liquid by filling the bottle with some other substance and making a cavity in between for the liquid.

Hachi Ko, Pilot (ATP) & Air Traffic Controller (FAA Terminal ATC-12). Answered July 11, 2017.

There is a limit at which a container can be effectively scanned for harmful substances. It’s not exactly 100ml, but it’s close. 100ml is a nice, round number chosen for its simplicity… it’s not likely to be confusing to anyone.

I have witnessed tests of security scans and you can definitely sneak harmful substances through in 1/2-liter containers, but it’s nearly impossible in 100 ml containers. Assuming everything is working correctly, you couldn’t, for example, sneak anything in using 5 100ml containers, but you easily could using a single 500ml container.

It has to do with density and volume. I’m not a chemist or physicist, but it was explained this way to me:

Could you find a single clear glass bead in a 100ml container? Pretty easily. But it would be extremely difficult to detect in a 1 liter or larger container, unless you really slowed down the security line for a much more intensive scan.

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  • I'm sceptical about the first point given that you can carry empty containers of other sizes - probably even a measuring cylinder to allow that precise mixing – Chris H Oct 5 at 15:31

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