All coins (and notes, medals and other numismatic items) have a story behind them. Not just the general story of designer, issuer, year, mintage, condition, catalogue value - but the much more personal story of where they came from and (in the case of a collector) why they were kept.
I'm mainly a world coin collector, but I have a few American banknotes. I've got two "fivers"; here's one of them. Technically, it's nothing much to write home about:
United States $5, Series 1981, Branch L (San Francisco), Almost Uncirculated, old sticky-tape stain on face, particularly near the left serial number. Catalogue value, maybe $10 or so.

But the story of why this particular note is in my collection is much more interesting than these bland statistics indicate.
It was Christmas Eve, 1983. I was eleven years old, and already a keen collector of anything numismatic. My family were making our slow progress from Vancouver down the west coast of the USA to Los Angeles, where our flight back to Australia would depart from. We were spending the Christmas weekend in Seattle. We didn't know anyone in that city, and were going to spend Christmas in a hotel downtown.
When we were dropped off at the hotel I remember seeing, out in the street, pieces of $1 notes. I had no idea why anyone would go around ripping up perfectly good money, but I knew that if I found enough pieces of a note (particularly the bits with the serial number) I could still get $1 for it. An eleven-year old doesn't get much spending money when travelling cross country, and $1 was worth scrounging for back in 1983. So after we checked into the room, I went back downstairs to reap my fortune. It was fine weather outside but below freezing, and all the heavy clothes were in the luggage somewhere, but I wasn't planning on being outside for long.
I was making good progress, too. But I was interrupted. There wasn't too much traffic around on Christmas Eve Saturday, but this big white limo driving down the street pulls up right next to me. The window rolls down and this elderly, well-dressed lady calls me over. She said, "Merry Christmas" and handed me an envelope. I thanked her. I don't know what she made of my Australian accent, but apparently after living for six months in Canada, I didn't have much of one left. The limo drove on. I opened the envelope, and inside was a Christmas card, with a brand spanking new $5 note taped inside it.
Apparently, I was the victim of mistaken identity. The nice lady had seen a young kid wearing nowhere-near-warm-enough clothes, scrabbling around in the gutter on Christmas eve, and decided to make the streetkid a beneficiary of her largesse.
My parents were embarrassed when I showed them and told them what happened, of course. We weren't that poor! But there wasn't much they could do about it - the card had no name or contact details by which to return the money. They let me keep it - my first fiver! I'd never actually owned such a high denomination note. And Uncirculated, too, or nearly so.
I didn't end up keeping the shredded bits of $1 note (my mum convinced me they weren't worth keeping), and I don't know what happened to the card, but the $5 note is still in my collection. A pity the sticky-tape didn't come off cleanly. Still, at least the stain reminds me which note belongs with this story.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis