A theory that can produce a statement that is not true, a theory that can predict something that does not in fact happen, or that predicts that something will happen but it does not happen, is not a correct theory. This idea has a big hole to fall into. Scientists can look at experiment after experiment, and they may always find that what theory says is true. Years may pass, and then somebody looks at one more experiment. That experiment shows that the theory is false, and every time they do that experiment it shows that the first time was not some kind of accident. The philosopher Karl Popper gave the example of people in Europe before 1492 who wanted to give scientific descriptions of birds. One bird they worked on was the swan. Somebody proposed the idea, "All swans are white." All of the swans that were checked in their century were white. When Europeans first sailed to Australia, the first swan that they saw was black. Suddenly the old scientific description of swans had to be changed.