It is one of the most commonly asked questions in the history of humour, a comedic puzzle that has spawned hundreds of variations.
But it appears we may finally have an answer to that most persistent of posers: how many people does it take to change a lightbulb?
In the case of the Scottish NHS, the answer is seven. Troubled insurance giant Standard Life manages the same job with three. BBC licence fee payers may be interested to know the corporation typically needs five people to change a bulb.
Joking aside, the quest to find out how many people large organisations need to keep the lights on started with a serious purpose.
When a fluorescent strip light in the office used by Dr Graham Ellis, a clinical research fellow in geriatric medicine at Glasgow Royal Infirmary, began to flicker, he decided to conduct an experiment in NHS efficiency.
He said: 'I wanted to find out exactly how many people it takes to change a lightbulb in a large inner-city teaching hospital. In the past I might just have wandered down to 'Bill in supplies' and got a bulb and put it up myself or have him do it. That would have been two people, including myself. This was a lot more complex.
This feels like a meme with legs. I see parliamentary questions about the number of people required to change a lighbulb in the Defence Materiel Organisation...