“Consulting a dozen or so recently published punctuation guides, I can report that they contain minor disagreements on virtually all aspects of the above and that their only genuine consistency is in using Keats’s poems as the prime example. Strange, but true. They just can’t leave Keats alone. ‘It is Keats’ poems (NOT Keats’s),’ they thunder. Or alternatively: ‘It is Keats’s poems (NOT Keats’).’ Well, poor old Keats, you can’t help thinking. No wonder he developed that cough.”
-Lynne Truss in Eats, Shoots and Leaves
Personally, I always use Jesus as an example when I try to teach about this, because it’s apt and also because once you bring Jesus into the equation–even when he probably wouldn’t have had an opinion on the matter–it lends the lesson a little more authority.
Also everyone knows it’s Jesus’s and Keats’s. Come on.