“Black Swan on Vacha reservoir, Bulgaria” by Kiril Krastev – Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons
At one time, Europeans thought that all swans were white as that was the only color of swan they had seen. However, discovery of black swans in Australia and blacknecked swans in South America proved that not all swans are white. The hypothesis that “all swans are white” was falsifiable (could be – and was – proved not true).
In scientific hypotheses, it only takes one “black swan” to prove the “all swans are white” hypothesis wrong.