Explain Montresor's play on the word "mason" when Fortunato asked Montresor if he was a member of the brotherhood.

When Fortunato asks Montresor if he is a mason, Fortunato
is referring to the secret society -- the Masons -- not to a person who literally works
with brick and stone for a living.  Fortunato is a Mason and he wants to find out (by
using the secret sign) if Montresor is too.

Montresor is
not a Mason, but he is going to be (later on in the story) a mason.  That's why he has
the trowel and shows it to Fortunato.  He is going to use the mason's tools to kill
Fortunato by walling him up.  So this is a foreshadowing of what is to come and
Montresor is enjoying himself by taunting Fortunato -- he's secretely telling Fortunato
"here's how I'm going to kill you."

Some people think that
maybe Fortunato kept Montresor from getting into the Masons.  They say that might be why
(or at least one reason why) Montresor wants to kill him.