Teaching moment fail

Last week when my family was in town, I had a conversation with Aiden (who is almost 4.5 years old) that went something like this:

Aiden: There are TWO grandmas here.
Me: Oh?
Aiden: Grandma, and ME.
Me: You can't be a grandma, you're male - a boy.
Aiden: Huh?
Me: You're a male and grandmas are female, so you can't be a grandma. You're a male like dad, grandpa, uncle Patrick and Bry. Grandma and mom and auntie are female.
Aiden: Male? I can fit in a mail box?
Me: No. That's a different kind of male, this is spelled differently.
Becca (from the other room): It has different letters.
Aiden: Letters? In the mail box!

...Somehow I did not anticipate him making those connections.