With all this talk of debt, currency devaluation, plummeting markets and creditworthiness, I've lately had visions dancing in my head of dollar bills being flushed down toilets and fed through shredders. Which got me wondering: what happens to our money -- our actual, physical money -- when it gets worn out?

Turns out, some bills wear out faster than others. The dollar bill has the shortest lifespan of all the bills, because it gets the most handling. The average life expectancy of a buck is just shy of 16 months. Fives last a bit longer: 22 months, give or take. Tens get about two months more than that, and twenties last around two years. Fifties and hundreds last considerably longer -- several years each -- and most coins last about 25 years, though occasionally you'll run across a penny from the forties or fifties; it happens.

So what happens to your money when it becomes unfit for use -- and who decides? Banks, mostly. They bundle torn and mutilated bills and send them back to the Federal Reserve. (If you have some messed-up bills on hand, you can exchange them for fresh bills at your bank, and they'll ship the bad ones back for you.) The Fed then decides whether or not the bills they've been shipped are actually ready for the trash heap. Generally speaking, if a bill is torn such that less than half of it is left, or mutilated so badly that its authenticity is questionable, then they're trash. They get shredded, bundled, and shipped off to landfills.

Most of that shredded money is never seen again, but some is kept and peddled to tourists. For instance, for just $20 you can buy $150 in shredded bills from DCgiftshop.com. (Maybe it's inflation, but that seems awfully steep.)