Marbles?
Sometimes I find myself talking aloud while doing something, without even paying attention to what I'm saying. When I think of a joke suddenly or an "amazing" pun, I say it out loud and then laugh and laugh.
My dad heard me a couple of times, giggling to myself, and every time he says, "She's losin' her marbles again."
When I'm reading a book, I find myself commenting on everything that I find funny or confusing. I read it to myself and work out what could be happening, and that's why I always prefer an empty room while reading.
Of course, I also hum to myself and sing, especially in the shower and while walking down an empty street.
It's as if my life is a movie, and I'm just laughing and commenting on everything that happens as if I'm watching it with some popcorn and licorice (only the red flavor, never the black ones).
Generally I consider myself pretty sane (key word generally) so if I do it, it's totally a-okay. ;)