Latex paint thinner
This is true,so not really a joke....
When I was a kid (early 70's) the local hardware store used to have old baby food jars filled with water at the checkout. Hand written on the jars it said "Latex paint thinner". My dad asked what they were for. The guy at the register told us that so many people came in asking for it the owner decided to start selling it. They would explain to people to just use water,but every once in a while somebody would not believe them,so they would hand them a jar of water.