You Know You're a Nerd When...

You know you are a nerd when you love knowledge and pop culture more than most people. Nerd is a proud term. Remember that.

You know you're a nerd when your dad hands you a scrap paper and it's a design plan and you name exactly what it is.

You know you're a nerd when you explain the meaning of a quote to your philosophy teacher by referencing spongebob

You know you are a nerd when you point out to a PHSE teacher that a point is invalid because "Milton was talking about the real Heaven and Hell, not as a metaphor. "

You know you're a nerd when you constantly develop your own theories, and test them.

You know you are a nerd when You draw up plans for a cardboard model of the first doctor's Tardis's control panel, using trigonometry and construction.
You then make said model.

You know you're a nerd when you design cars and make wooden models.

You know you're a nerd when you forget to feed your dog because you're on the computer.

You know you're a nerd when you designed plans for a flying car as a child that turned out to be very similar to what scientist are trying to do now.

You know you're a nerd when you're scanning Facebook, see that your high school crush's status is "[Name] is missing his electron", and your first thought is "Oh no, did he break up with [high-school sweetheart]?!?!?!?!"

That actually happened, and yes, they did break up and it was sad.

You know you are a nerd when you automatically argue against anything, even if you agree with it.

You know you are a nerd when you make a hypothesis about the electromagnetic spectrum, test it, and then disprove it.

You know you are a nerd when your occupation on YWS is "Nerd Extraordinaire"

You know you're a nerd when You come to this forum. :p

Original @Deadman
You know you are a nerd when you back up your arguments with in depth psychology you made up on the spot, and it works.