There is already an Instructable on making a cryptex published lately, but I wasn't satisfied by that one. That cryptex was held together on the outside, like this. I prefer my cryptex held together from the inside, like this. That is by the way the 'real' cryptex, like it is used in the film. Those are harder to make.
A cryptex is a kind of lock with a password, that holds a roll of paper. Author Dan Brown said in his "Da Vinci Code", a book from some years ago, that Leonardo da Vinci invented it, to let messengers carry secret papyrus rolls that the messengers themselves may not read; only the sender and the recipient know the password. If the messenger would smash the cryptex to read the message, a vial containing vinegar which was rolled inside of the papyrus roll would also break even earlier than the cryptex itself, the vinegar would spread over the message, disolve the ink, making it unreadable.
The term 'cryptex' is actually made up by Dan Brown. I don't even know for sure that it's true that Da Vinci invented it, he never actually made one, he would have made a blueprint in his diary, but I never found that page of his diary(so I made my own blueprint, see the next step). What I do know, is that the vinegar thing doesn't work. It doesn't disolve the ink so much that it gets unreadable, it becomes only a little more vague. I tested it with Black Indian ink, several kinds of pen inks, marker pens, ecoline, pencils, and some more; it never worked, just like Wikipedia says.
In the Da Vinci Code, the cryptexes were used different than how they were meant by Da Vinci, in the story they use it as a part of a big puzzle, the answer to a riddle is the password of a cryptex, which contains another riddle that leads to another cryptex, etc. To my opinion the use of cryptexes in such a way is a bit ridiculous, the characters in the book could've just put the cryptex in the freezer so that the vinegar gets frozen, then they could saw it open without danger. But they didn't think of that easy solution in the book.
In the comments below, 'cavingboy92' came up with this genious way to make use of the vinegar vial: on the paper around the vial, don't write with ink, but also with vinegar. If you want to read the message, pour a PH indicator on it(like red cabbage juice), and you can see the text. Or you can just hold it under a low heat source like a light bulb, then you can see it too, because the acid will oxidate.
If the vial breaks, the piece of paper gets all drenched in vinegar, so it's absolutely sure that it's unreadable!
Here is the proof that it works, tested by cavingboy92.
You could also just put the same inkin the vial as the ink where you wrote with on the paper, that would also destroy the message.
If you want more ways on how you could use a vial-break-delfdestroy mechanism, read throught the comments below, there are a lot of people giving suggestions for it, and also on improving the cryptex.
Finally, if you disolve a lot of salt in the vinegar, I think you can lower the freezing point of it so much that it can't get frozen in a normal kitchen freezer, then the freeze method also won't work so easily, you would have to find a special freezer in a lab or something.