Given a set of axioms S, there are some facts that S tells you nothing about. Assume that the system knows or can derive everything that is true. Then anything that the system can't prove is false. John put on his coat and left for work. The closed world assumption states that everything is false unless explicitly stated or derivable.
Birds can fly.
A turkey is a bird. Therefore turkeys can fly. The turkey on the Thanksgiving table is a turkey. Therefore the turkey on the Thanksgiving table can fly. This is wrong.