There are 100 lightbulbs in a room, each with it’s own switch in the off position (all lightbulbs work and start off, no funny business). There are also 100 people numbered from 1 to 100 standing outside the room.
Each person enters the room one at a time and flips the switches of certain lightbulbs.
The 1st person flips the switch of every lightbulb
The 2nd person flips the switch of every 2nd lightbulb
The 3rd person flips the switch of every 3rd lightbulb
All the way until the 100th person walks into the room and flips the switch of (obviously only) the 100th lightbulb.
What lightbulbs are left on after all 100 people have entered?
Note: I’m not sure where I found this puzzle, but I certainly did not come up with it myself. I just remembered it and decided to share it with you all. If nobody gets the answer within a week (I’d be less surprised to see pigs fly), I will post the answer.