Does this need an apostrophe?

Most of the time, when people ask this question, they are talking about possession and ownership. We know how to use apostrophes for words like don't, didn't, and can't (showing contraction). Showing possession with an apostrophe is more difficult for a lot of us. We are just not sure what needs an apostrophe. Bob's car might make sense, because Bob owns his car. But Bob also owns his knee, as in Bob's knee hurts. Even more strange, Bob owns his plans: Bob's dinner plans were ruined. He also owns his height: Bob's height is 5'8". A house can own its gutters: The house's old gutters fell off. Even an idea can own its own time: An idea's time has come. The key to deciding if the word takes an apostrophe is ... does it own the words that follow it? If a sentence reads "Kids on the playground like to test the swings height," the possessive in the sentence is not kids. The word kids is followed by "on" and the kids don't own the word "on." However, the word swings is followed by the word height, and the swings own their own height, so swings takes an apostrophe. The only problem left is to decide where to put the apostrophe! Is swing's or swings' correct? Check for the solution in my post about the thumb trick for apostrophes.