Or any sort of recorder really! Sometimes, I've been in the car when an idea has popped into my head and I've often grabbed the mobile phone and used the crappy voice recorder / memo function to whistle in the idea!
I'm not sure I'd agree with the notion as expressed earlier of "inspiration being overrated" especially when Franksspace made the highly valid point of emptiness or silence being an excellent motivation. Many people might find emptiness or silence anything but inspiring, but the fact remains that it can strike when you least expect it and in my case, whether you want it to or not!
Perhaps "inspiration" as a concept (particularly in reference to composing) has been vaguely articulated or badly expressed or something, but I wouldn't say it's overrated as it still amazes me the with the places and moments it chooses to strike.
Case in point, I'll tell you something that happened at school the other day. I was tidying up and closing doors after a class had left. After wheeling the xylophone back into its storeroom, I dropped my whiteboard markers onto the keys whilst reaching up for a book on the shelves behind. The marker pens bouncing on the xylo keys created a pretty cool little riff. I dropped the book (literally) and immediately worked out what the pens had "played" (!!

!!), grabbed the nearest sheet of manuscript and wrote it down. It's now being used in a little piece for the whole class to play.
Sometimes thinking about it too hard can be the worst thing. It's often best to wait for it - most times you'll find it won't take long.
