For next time Lenoir, you'll know that the apostrophe is possessive, i.e. denoting belonging to someone. I
think (but I may very well be wrong on this) it's a contraction of "his" and "hers", as in "Lenoir, his book" becomes "Lenoir's book". If something belongs to a group, like a group of magicians, you no longer need the "s", so it becomes "the magicians' box", meaning the box belonging to the magicians... because "magicians's" would be incorrect.
The reason it becomes complicated is because it's the opposite with "its/it's"; "its" is possessive (belonging to it), "it's" is a contraction of "it is" and not anything else. Knowing this, you'll now be astounded at how often you see an apostrophe in the word "its" where none is needed!
