For decades (1940s through the early 1990s), the Number Five Crossbar Switch was a mainstay of the Bell Telephone System in the United States. Originally designed for smaller central offices with between 3000 and 5000 lines, it was arbitrarily expandable and later used for exchanges with tens of thousands of lines.
Electrical engineers have learned from experience that what makes electronic components work is magic, and this magic is embodied in magic smoke infused into the devices when the elves create them. When a device fails, it releases the magic smoke and doesn’t work any more. Repair consists of figuring out what caused the smoke to be lost and replacing the component(s) that lost it with new ones containing magic smoke.
As is well known, the Telephone Company did everything differently. Nothing they made resembled anything else used by other companies or in different kinds of electronic gear. So, instead of magic smoke, a Number Five Crossbar installation contained a multitude of sealed relay cans that did the switching work. When things went wrong, these did not release magic smoke but instead dribbled out green goo, which clung tenaciously to anything it touched, leading those who encountered it to suspect these components were blessed in the factory by a minion of Moloch rather than the benign elves of electronics.
Here is what happens in the aftermath of a release of the green goo.