For ages I've been looking for an old-fashioned washboard for washing "smalls" in a bowl, but they are impossible to find in the shops here in England. Maybe elsewhere in the world people still use them! I could buy one on the internet, but they are all shipped from the USA or the Far East and take ages to reach the UK.
And then I happened to be making a cup of tea when my eyes wandered to an empty soup can on the dresser. (I use old cans for storing pencils, pens, paintbrushes and so on.) Immediately I noticed the corrugated nature of the tin can! Could I use that, maybe? So I quickly removed the other end from the can to form a cylinder, stripped off the original label (the can shown once contained chicken soup), and gave it a go.
It works! Maybe the ripples aren't as deep as on a proper washboard, but they certainly do work to scrub out sweat stains (or other nasties!) from small items of clothing.
So that is the sum total of this Instructable! Nothing much more to add. Just find yourself a suitable can, eat the contents (not if it's dog food), take off the bottom and give it a good wash. That's all, folks!