You've no doubt had this happen to you: you've got ONE dollar bill in your wallet and it's a little warn or wrinkled. Sure enough, the stupid vending machine won't take it and just keeps spitting it back out. What do you do now?
Scrape it back and forth on the edge of a desk like you're shining a shoe? Turn it over and try the other side of the bill? Moisten it a little? Nah...Here's a little trick to make it work 95% of the time. Lay the dollar face down and fold it length-wise by bringing the top edge down to the bottom edge. Make the crease as tight and as sharp as you can.
When you turn it over, the crease will be running left to right folding Washington's face in half like this:
Now, unfold it so it looks like a normal dollar again but don't try to flatten out the crease. Leave it so it's still there and resembles a tent. Go and insert the dollar into the vending machine (face up) with the sharp point of the crease facing up and the dollar will now work. Not sure why exactly, but it changes the surface in a way that allows the "reader" in the validator to see the dollar better--especially an old wrinkled up one.