I can't seem to stop thinking about nested systems, like matryoshka dolls, and their cousin, recursion, which I see as nesting with a chance of looping. Examples:

  1. A list of list of lists
  2. A thought experiment about thought experiments - haven't located one yet, may have to make my own
  3. An island in a lake on an island in a lake on an island 
  4. Acronyms that include other acronyms
    FCAA = "Funders Concerned About A(quired) I(mmune) (D)eficiency (S)yndrome" [nesting]
    GNU = "GNU's Not Unix" [recursion]
  5. The Xzibit "Yo Dawg" meme - see below

Also, when you perform a google search for "recursion," it helpfully prompts you:

Did you mean: recursion