If you want a role-play reason -- they aren't physical object. They are a pact (agreement) between you and the Higher Powers to transport you back from where you are to a specific location. If you have anything in your pack it is the equivalent of a subway ticket or perhaps an airline boarding pass. The paper is all the same q.
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Remembering Tyneetryk
Phaedreas Tears - 15 years old and first(*) of true neutral guilds in Atys.
(*) This statement is contested, but we are certainly the longest lasting.
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