Water bottles containing water are not allowed at the airport beyond the security checkpoint. Passengers can keep them in a checked luggage, carry water bottles up to the point before the security check or buy them from the airport stores behind the security checkpoint. One can even carry an empty bottle in hand luggage and fill it later after security check at the airport but never ever a filled water bottle is allowed to be carried through the security checkpoint. While many of you would have wondered why that happens, chances are that many of you do not know the actual reason.
As per answers to a question asking the rationale behind the rule being followed the world over, it is because bottles carrying more than 3.4 ounces of liquid are a security threat as they may contain explosives in a liquid form. Since one can always fake a sip, allowing water bottles even after passengers verifying bottles as that of being water only and not explosive cannot be a foolproof solution.
The rule was implemented after US intelligence agencies foiled an attack planned by terrorist group Al-Qaeda.
Upon reaching the spot where the explosives were being made, US authorities found that the explosives were liquid hydrogen bombs and were being packed in mineral water bottles. So an alert for ban on water bottles beyond the point of security check was issued all over the world with immediate effect and is being followed ever since.
It is believed that had the attack not been averted , it would have been one even more worse than the 9/11 attack!