Imagine a ladder with one rung, which is not really a ladder, but imagine it. If you loop a piece of string around the rung exactly once and tie it, you can't remove it from the ladder (assuming the sides of the ladder are too long to do so). But if you remove the one rung, you can.
Now suppose the ladder has two rungs, and you want to loop a piece of string around the two rungs so that you can't remove it, but removing any one rung allows you to do so. Again, the sides of the ladder are too long to pull the string over.
How do you loop a piece of string around two rungs so removing either rung frees it?
If you can do two rungs, try for three rungs.
How do you loop a piece of string around three rungs so removing any rung frees it?
The general case is left for those seeking a challenge.