I used to be a banker

This classic one-liner about a former banker relies entirely on the delightful art of the pun. The humor mechanism here is pure wordplay, hinging on the double meaning of the word "interest." On one hand, "interest" is a fundamental concept in banking, referring to the money paid regularly at a particular rate for the use of money lent, or for delaying the repayment of a debt. It's the bread and butter of financial institutions, the very core of how banks operate and make money.

But then, the joke cleverly pivots to the other meaning of "interest": a feeling of wanting to know or learn about something, or an activity or subject that one enjoys and gives time to. When someone "loses interest" in a job, it means they've become bored, disengaged, or simply don't care for it anymore. The unexpected switch from the financial, numerical world of banking to the relatable, emotional experience of waning enthusiasm is what sparks the laugh.

The joke works so well because it takes a rather dry, technical concept from the world of finance—which many people interact with but might find a bit dull—and injects it with a relatable human emotion. It's a quick, clever jab that turns a professional predicament into a witty observation, proving that even the most serious professions can be ripe for a good linguistic twist. Simple yet sophisticated, it’s a perfect example of a pun well-deployed.