Contented (resplendent adjective), content (modest adjective), and content (the noun)

What's the difference between the adjectives content and contented—as in I am content/contented? This is an oft-recurring discussion topic among people who are learning English as a second (or third or fourth) language.

If English is your native tongue, you instinctively know the following:

• Before a noun, contented is the only adjective that sounds natural and correct, for example, She is a contented baby.
• Anywhere else, either adjective will work: for example, She was no longer content.
• The two words are used in slightly different ways: you are content with something and contented by something.
• An excruciatingly subtle difference of meaning could be debated, for example, I become contented if something converts me from a state of discontent, but to be content is a steady state. (Mind you, nobody cares!)

So contented and content are virtually interchangeable as adjectives.

Naturally, Alice and I find the multipurpose adjective contented more appealing. Why? Let me count the ways!

• Contented has charming manners—we can take it anywhere.
• Contented is more dynamic than its inert cousin content: a mood-changing event has happened here.
• The amphibrach rhythm of contented is far more impressive than the common or garden iambic content: our favourite is a word with three syllables, hard for any martial artist to topple.

And as for the noun content—meaning the thing contained—that's another conundrum, one that I peck away at year after year. Where did it come from? Why do we need it? Who uses it? Who despises it? More later...

P.S. The mystery is solved. MS Word autocorrect is the culprit.
Image of smiley face inflatable toy: (c) unknown, sorry.