Six situations
“If men define situations as real, they are real in their consequences.”
W. I. Thomas and Dorothy Swaine Thomas, The Child in America (1928). She co-wrote the book and the idea; the theorem is usually named for him alone.
Read it carefully. A definition does not bend physics. The rumor never drains the vault — people drain the vault. Beliefs become real by changing what people do, which is why this is a micro-level insight and why it belongs to symbolic interactionism, not to functionalism or conflict theory.