Recursion, and also how fun it is to destroy the meaning of a word by repeating it.

On my bike a few days ago (surely the last time I'll ride in this cold) I was thinking about thought,* and then thinking about how I was thinking about thinking, and then I thought: you can only get about two iterations out (thinking about thinking about thinking) before you're not really thinking about anything, because all you're thinking is, "I'm thinking about thinking about thinking." Unless you have the thought that you're not really thinking anything when you think about thinking about thinking, which is actually a thought about thinking about thinking about thinking.

I think.

*thinking about thinking happens naturally. For example when Hamlet says, "Haste me to know't that I with wings as swift/ as meditation or the thoughts of love/ may sweep to my revenge," he expresses a thought about thinking, i.e. it's pretty swift.