Stretch your brain

Yesterday during lunch, (alamak, have to solve math problem for him again) my boss sent me this email:

The following is among 10 math questions given to Sec One students at Hwa Chong about a week ago. Can you solve it ? ( I can’t …… )

• To celebrate the 2009 Math Festival, a teacher set up 2009 numbered lights in a row
• The lights are originally switched off
• The first student went up to all the lights that are multiples of 1 and switched them on
• The next student then went to switch off all the lights that are multiples of 2
• The third student then went up to the lights that are multiples of 3, and switch them on if they are off, or switch them off if they are already on
• This process continued until the 2009th student, who then switched on or off the last light
• The teacher then asked another student to count the number of lights that remain switched on
• Q : How many lights did he count ?

Try it, it is not that difficult as it first seem to be.