<human>: Give me a shortened version of the text

I have a fairly uninteresting job and to help me along I do a lot of useless business in my head, e.g. today I calculated how old I am in days, double a random fractions, and tried to come up with an educated guess as to why the suns corona is hotter than it's surface. I'd just like some suggestions for things I can do in my head like this.

I can't really listen to an mp3 player, and I don't have company for most of the day, so that's limiting... Are there maybe any small books or cards of riddles and stuff I can keep in my pocket to pull out at glance at once in a while?

Obviously I could just get a more interesting job, but in the meantime I'd like to have some ideas.
<bot>: I'm looking for anything to entertain me mentally that I can do mostly solo and in my head. Not too much writing, reading, or listening.