A school has 100 lockers and 100 students. All lockers are closed on the first day of school. As the students enter, the first student, denoted by S1, opens every locker. Then the second student, S2, begins, with the second locker denoted L2, and closes every other locker. Student beings with the third locker and changes every third locker (closes it if it was open, and opens it if it was closed). Student S4 begins with locker L4 and changes it every fourth locker. Student S5 starts with L5 and changes it every fifth locker and so on until student S100 changes L100.
After all the students have passed through the building and changed the lockers, which lockers are open?