The doctrine uses a simple biological analogy to protect constitutional rights:
If police perform an illegal traffic stop and find a key to a locker, any evidence found inside that locker is "fruit" of the illegal stop. Unless one of the exceptions applies (like proving they were already planning to search that locker), the evidence from the locker will be suppressed in court. Fruits from a Poisonous Tree
Justice Felix Frankfurter coined the specific phrase "fruit of the poisonous tree" in the 1939 case Nardone v. United States . The doctrine uses a simple biological analogy to