Did you hear the rumor about butter?
Why this joke works
Keeping a Secret Sounds Like Leaving the Toast Plain
To spread a rumor is to pass it along to other people. Butter spreads when someone smooths it over bread or another surface. The opening offers gossip, then the response refuses to share it using a verb that belongs equally well in the kitchen. The withheld information is not missing from the joke: withholding it is the joke. A listener expects news about butter and receives a small demonstration of the word that could describe using it.
A closer look
The Refusal Does More Than an Answer Would
A made-up rumor would add detail but weaken the tidy ending. “Well” gives the speaker a moment to reconsider telling it, creating a tiny conversational reversal. Sound as though you have decided to be discreet, then stop. That leaves the audience with both the imagined secret and the oddly untouched spread.
Fact behind the joke
Churning Changes Cream Into Butter
USDA’s butter-grading information describes cream being churned until its fat separates from the liquid buttermilk and is worked into a semisolid form. Butter’s texture is produced by reorganizing components already present in cream. It is not simply cream that has become thicker through ordinary stirring, nor is it made by adding yellow coloring to milk.