My Calculus teacher sends out extra credit questions through my school's Online Blackboard thing. First 10 students to answer the question get extra credit and I need some bad!

I should know this answer, it's not even Calculus but I'm so burned I can't think. Can someone just help me get started on it?

In an exceptionally long corridor in a factory, there are one thousand windows along one wall. Coincidentally, there are exactly one thousand workers in the factory. The foreman orders the first worker to open the blinds on every window. Then, he orders the second worker to close the blinds on every second window. Then the third worker is told to go to every third window, and close the blinds if they are open, and open the blinds if they are closed. The fourth worker does this for every fourth window, and so on.

After all 1000 workers complete the process, how many blinds are open?