hard disk used up

i just got my 12" ibook about 2 weeks ago..its the cheapest version with 40gb hard disk.. with applications and all, i've used up about half of my hd. i download a lot, and after i download i put everything in an external hard disk. the problem here is, my laptop hd keeps on decreasing with no apparent reason, even after i finish my download and moved my files into the external hard disk. it gets restored after i restart the computer. i wouldn't have minded if it were just a couple of megabytes. just to give you an idea, before i restarted, i had 19.8 gb free..after i restart my computer, it says 21gb free.. so what's up here?

OSX occasionally does that on my G4 tower as well. I'll delete 1 or 2 Gb of stuff and the space won't show back up as available until a reboot.