Writing music without instrument?

Sometimes I sit at work, and some catchy melody or rhythm pops in my head but never know what to do with it? Or how do I remember it when I get home to try it out on my guitar? Then I just hit n blank when i want to play something. . . Any tips would be much appreciated.
I dont have much knowledge in theory though. . .

learn how to write and read music notation. I heard some people are so good that can just pen it down with out playing it. but rhythm would be easy to write down(at lest the gist of it) as you could get anything to pound on. Or you could take my approach if it good enough I'll remember it if not then who cares, but i have the same problem.

Get a recorder, and hum the melody or what-have-you into the recorder. That way you can play it back whenever.

Hmmm, I think the recorder method can work! thanks will give it n try!