A popular phrase to say when you are currently in denial that what you have been served on your warm toast is not butter, but rather,
margarine.
The phrase became especially after a brand of margarine used it as their name, to show just how their margarine was more similar to
the real thing, however, now it's pretty normal that margarine tastes
like butter.
Person A: This toast is so tasty. What is
the brand of butter you use?
Person B: That's not butter, that's
margarine.
Person A: What? I can't believe
it's not butter!