I've started telling everyone about my new diet.

This one-liner gets its chuckle from a clever bit of wordplay, specifically a pun. The humor hinges on the double meaning of the word "spread." When someone talks about "spreading the word" about their new diet, they mean they're telling people about it, sharing the information. The punchline then takes a delightfully literal turn, contrasting that informational act with the physical action of "spreading the butter," which is a common culinary practice, often associated with adding richness—and calories—to food.

The joke taps into the very relatable world of dieting, a topic many people have experience with. Diets often involve making conscious choices to limit certain foods, and butter, while delicious, is frequently an item that dieters try to consume in moderation. By setting up the expectation of sharing news and then twisting it to a common food item that might derail a diet, the joke creates a lighthearted and knowing nod to the everyday struggles and temptations faced when trying to eat healthier. It’s a quick, witty way to suggest that sometimes, a good conversation is indeed more satisfying than a buttery indulgence.