"This is not mayonnaise, this is yogurt!"

Once a dear friend told me that we laugh because something happens or is said that contrasts what we are anticipating, what we expect to come next.

for a trick of the eye, and hopefully a laugh, or at least a smile-check out Trompe l'oeil...

*note, i consume neither mayonnaise nor yogurt...it is a joke from czech language class in prague*