Slam!

Here's a puzzle that piqued my interest a little while ago:

There are 100 doors in a row, that are all closed. Now you make 100 passes through the doors. In the first pass, you toggle every door. In the second pass you toggle every second door. In the third pass, you toggle every third door and so on. Now after 100 passes how many doors are open and how many are closed?

(Toggle means to close a door if it is already open , or open it if it is already closed)