Joke stems from the fact that the French term "Papier parchemin" could mean a scroll, or could mean parchment paper. I'll call it "parchment" below.
G: If what I just read is right, we have a chance to counter the plans of the dark wizard. Frodon, pass me the parchment.
*thunk*
G: No, not the parchment, the parchment.
F: But Gandalf, this IS parchment.
G: Not the parchment, the PARCHMENT.
F: I've just given you the parchment, Gandalf!
G: Frodo you little snotrag! We're here to counter the plans of Sauron the terrible, not to make fucking fruit tarts!
F: Sorry Gandalf. Then may I suggest instead tinfoil?
G: GODDAMMIT FRODON!!