Tarski's undefinability theorem

Tarski's Undefinability Theorem says that you can't define something using only words from the same language it is written in. It's like that old saying about how you can't explain what a word means without using that same word. For example, if you were trying to explain what the word "butter" means, it wouldn't work if you said something like, "Butter is a thing that is butter." That wouldn't make any sense!