Apostrophes are used to show ownership. If the thing that owns something is plural, then we are supposed to put the apostrophe after the "s". For example:
"Three cats' toys are all over the floor"
But what if the plural thing does not end in an "s". For example "oxen" is the plural of ox but does not end in an "s". So if we want to say "the poop from the oxen is all over the place", how do you say this using an apostrophe?