What is a trillion?

With all this talk of credit crunch and the BBC News endlessly going on about whether the American Senate might or might not vote $700 million/billion - I've heard both reported - to cover the deficit caused by mortgages that never had a hope of being repaid, I remembered Bill Bryson.

In 1999 he published Notes From A Big Country and he did a piece on the economy. This is what he says: "No matter where you turn with regard to America and its economy you are going to bump into figures that are so large as to be essentially incomprehensible." He goes on to quote the annual gross domestic product as $6.8 trillion, the federal budget at $1.6 trillion, the federal deficit around $200 billion.
Do you have much idea of what a trillion is? I don't. It looks like this:
$6,800,000,000,000. I'd like to work out what % $700 billion/million is of that total, but my calculator won't take that many numbers.
Snookered.