HTML element: summary

<details>
  <summary>Foo</summary>
    <ul>
      <li>The original, the alpha, the one that started it all
      <li>Almost certainly came from the U.S. Army slang FUBAR ("Fucked Up Beyond All Recognition") → shortened forms like FOO appeared in military jargon already in the 1940s
      <li>Became widely popular in computer science / hacker culture in the 1960s–1970s (MIT, Bell Labs, early ARPANET crowd)
      <li>Default "first" metasyntactic variable — when you need one throwaway name, you start with foo
    </ul>
</details>
<details>
  <summary>Bar</summary>
    <ul>
      <li>Almost always shows up as the second metasyntactic variable
      <li>The natural companion to foo — like salt to pepper
      <li>Sometimes jokingly said to come from "foobar" being treated as one word → split into foo + bar
      <li>Extremely common in teaching examples, Stack Overflow answers, blog posts, GitHub gists…
    </ul>
</details>
<details>
  <summary>Baz</summary>
    <ul>
      <li>The traditional third metasyntactic variable
      <li>Used when you need one more name after foo and bar
      <li>Sometimes people continue the sequence: foo → bar → baz → qux → quux → quuz → corge → grault → garply → waldo → fred → plugh → xyzzy → thud…
    </ul>
</details>