Onion In The Varnish

In The Periodic Table, PrimoLevi tells a story that happened when he was working in a varnish factory. He was a chemist, and he was fascinated by the fact that the varnish recipe included a raw onion. What could it be for? No one knew; it was just part of the recipe. So he investigated, and eventually discovered that they had started throwing the onion in years ago to test the temperature of the varnish: if it was hot enough, the onion would fry.

(an example of OldRulesWithForgottenReasons)