When making a word that ends in an “s” possessive, teach your children they can just add an apostrophe or add an apostrophe and an “s.” For example, “Platypus' front foot” is just as correct as “Platypus's front foot.”  Don't let them be fooled, though, by plural possessives ending in “s” like “mosquitoes” or “burritos.” Those words get an apostrophe only, as in “mosquitoes'” and “burritos'.”