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!