Where does the apostrophe go?

So let's say Bob owns a car, and you want to write that it is Bob's car. Bob takes an apostrophe and an "s" to show possession of the car. But Bob is a pretty simple example. What if you are writing about the mailbox that belongs to Mr. and Mrs. Jones next door? Or what if you are referring to the goat farm that Silas owns? Well, try my thumb trick to figure out where the apostrophe goes. Write the two choices down: Jone's mailbox and Jones' mailbox. Then in each choice, stick your right thumb over the apostrophe and everything to the right of it. The first choice will leave you with Jone. Hmm...that looks funny; definitely not their last name. The second choice turns out as Jones, and that is their name, so you have found your answer. Try this again with Sila's goat farm and Silas' goat farm: you discover the same secret, that the name won't make sense when you cover up the apostrophe on the incorrect choice. My thumb trick has never failed me. It even handles confusing situations like this: Toys covered the girls' floor all the way to their bunk beds. Is girls' floor or girl's floor correct? You can tell there are two or more girls because "they" have "bunk beds" so if you use the thumb trick, you want to see the word girls before the apostrophe. Who knew your thumb could be so smart!