The question of why peanut butter jars have a shape that makes it difficult to scrape them completely clean is one that has been vexing us for quite a while, to the point that we’ve put serious thought into our own solution. The Guardian recently put the question to its readers, who proposed a number of reasons. The most logical is that an inward curve at the top of the container makes the rim stronger and less prone to breakage when you twist the lid. The most fanciful is that the nearly empty peanut butter jars were used by miners in Kenya to lure and capture sapper ants that sometimes had flecks of gold dust clinging to them. (The only parts of this story that I can verify is that in Kenya, there is a town called Migori and there is a gold mine there.)
The most practical answer is not an explanation at all but the recommendation of a Dutch kitchen tool called a “flessenlikker,” designed to scrape food from the bottom of jars. And it is available online for Americans!
I do still like the idea of the peanut butter jar that pays for itself by capturing gold ants, though.