Oh wow, I can actually answer this with a specific event.
It was a few months ago in my dorm room. I was alone, and I couldn't sleep. So I just laid there staring at the wall, and then I started to think about life and death and why we're here... etc.
Somehow I came to the conclusion that the only thing I know for sure is that I don't know anything, and that proves that I'm a sentient being and not some organism.
And then later I figured out that Tolstoy said the same thing. I dunno, I'm sure I didn't hear the quote before, but either way that was pretty cool.