Why can't we bring water in an airliner

I wonder why do passenger prohibited to bring water aboard an airplane.  After thinking thoroughly, I have 3 possible reasons:

1. Safety reason: some liquids for bomb making might be disguised as water and will not show up on X ray scans, therefore limiting water will reduce likelihood of terrorism.  Well, a nice reason, however this reason is flawed since liquid bombs could be easily smudged by the airport workers.  
   
2. Convenience reason: Water is likely to spill on the hand carry therefore disturbing others.  This is ruled out since we can bought water in boarding room and bring it aboard the plane!  If some states the liquid is contained in plastic bag, we can always open it aboard.  
3. Economic reason: to bring more customers to buy expensive water in the airport's boarding room store.  Therefore generating profits and tax for the airport and the stores.  This is the most make sense reason after all.

How did I ruled out the first reason: (this is important for airport operators and airliners), water and alcoholic beverages could be easily made into explosives.  We could bring the tools to extract the explosives substances on water without the security noticing it.  And then build the extractor aboard the plane. 

Thus creating a mini bomb, a very low explosives that won't destroy the plane, however enough for scaring everyone on board!  I won't tell you what to bring, how to do it, and I will not hijack any plane!  This is a warning for airport operator and airliners!   The tools are available on daily live and did not appear very scary. 

Anyway, did you know that it is perfectly safe to bring bomb on board an airliner?  Bomb won't explode unless being deliberately triggered.   Here is the proof: the jet fighters and bombers all around the world brought explosives onboard everyday!  And they are save on board and did not explode!  

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