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I can't take a bottle of water on board, but I can drink it in front of the guard and get on board myself.
If this was explosive, it's now inside me, I could still ingest something to start the reaction. But heaven forbid I have a bottle of water....
And let's not forget, after security there is duty free liqueur stores that you can carry on bottles of booze. Did you know alcohol is explosive? Apparently airport security doesn't to miss such an obvious thing.

And as XKCD points out, a rigged laptop battery can explode, but be undetectable.
http://xkcd.com/651/