I'm pretty sure they are. They have never once left their house during the day. At night they leave all their lights on until morning, during the day they pull all the blinds.
And they act weird at night. The other night I saw one standing motionless in their backyard staring at the sky. He stood there, still as a statue for hours.
I am pretty sure they are vampires, I don't know how to test it though.
(no, this is not a movie reference)
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Just get into an argument and say "bite me" when you're done and just see how it goes from there.