I looked over the edge of my book and decided to leave Al and SW to play their silly games.....
The aim is to flip your bottle cap so that it lands on top of your opponent's bottle cap with the picture side uppermost. Any overlap, with the the pretty side of your cap showing is a win and means that you have won that round and so take possession of both bottle caps. Play continues until one player has no more bottle caps left. You can start with any number of bottle caps - but three each seems to make for a good game.
If your bottle cap goes off the edge of the table (or other improvised playing surface) then you have lost your turn and forfeit your bottle cap to your opponent.
If you flip your bottle cap so it lands on your opponents cap as shown in this photo then you have lost your turn and forfeit your bottle cap to your opponent.
It takes a while to master the knack of flipping the cap but as I watched Al & SW play against each other it was noticeable that they were drawing a small crowd of boys who were watching avidly. In the absence of smartphones the game spread like wildfire through the group. Whenever we had a few boys, a few bottle caps, a few minutes to spare and a flat(ish) surface a game would be started. It is a great way to pass a few minutes and does get quite competitive.
Since returning home, I've taught it to my family, in fact Husbando and I spent 10 minutes playing it this evening while waiting for our curry to be served. He's got the hang of flipping at last and we are pretty evenly matched now. I love the low tech nature of this game, easy to play anywhere with minimal resources... and it is surprisingly addictive!
* If you are playing against a small child then you can let them use a finger rather than their thumb.