Formal fallacy

A formal fallacy is a mistake in reasoning that's based on the way an argument is structured. It's like having a puzzle with the wrong pieces, so the answer is wrong no matter what. For example, if you say "every person over 50 years old likes ice cream" and then base your conclusion on that statement by saying "so, I like ice cream because I'm over 50," that's a formal fallacy. Even if it's true that you like ice cream and you're over 50, the two were not connected in a logical way.