The chemist-turned-cool-Holocaust-memoir-writer Primo Levi has a story about a time he was working at a chemical plant that made varnish. He was surprised to find that in addition to the chemicals he expected, the varnish formula also called for a raw onion. At first, he could find no reason as to why a raw onion had to be added; there wasn’t anything in onions that was needed in varnish, and even if there were, a single onion would be too little for a large industrial vat.
After doing a little research, Levi found out that his predecessors used to toss an onion into the varnish as a simple and inexpensive way of testing its temperature. If the mixture was hot enough, the onion would fry. With modern equipment, the need for the onion had vanished, but for reasons they no longer knew, it had become part of the recipe.