Because the shackles the Order had placed me in were not intended to be removed, they did not have any typical mechanism to take them off like a lock, keyhole or such.
"I have a small vial of liquid in my right nostril. If you could take it out and pour some of the contents on each of the restraints, that'd work..."
Thus, I needed a sure-fire way of getting out of them.
As for why I had stored it up my nose, well, where else was I going to store it?
I knew that my entire body, including my mouth, was going to be searched, so the option of hiding it anywhere I normally would was automatically gone.
I also couldn't hide it in my ears because of the plugs I knew they were going to put in there during the kidnapping.
I could have swallowed it and forced myself to throw it up when needed, but not only does it come with the unnecessary risk of the capsule possibly leaking inside my stomach, but it also requires me having to throw up.
