My coworker told me he was going to start a business making invisible ink.

This joke hinges entirely on a delightful bit of wordplay, specifically a pun that's so good, you might not even "see" it coming! The humor comes from the double meaning of the phrase "I don't see that working out." On one hand, it's a common idiom for expressing skepticism about a venture's success. On the other, it's a literal statement about invisible ink – if it works as intended, you absolutely wouldn't be able to see it, making a business based on it a bit of a paradox.

Invisible ink itself has a fascinating history, dating back centuries to ancient times when people used various substances like milk, lemon juice, or even urine to write secret messages that would only appear under heat or chemical reactions. It's been a staple of espionage, secret societies, and countless childhood spy games. The inherent nature of invisible ink is to be undetectable, which makes the coworker's business idea inherently funny, and the punchline a perfectly crafted, dry observation that cleverly plays on this very characteristic. It’s a joke that proves sometimes, the best humor is right there, even if you can't quite see it.