there are different ways in which it can be done. the easiest way to do it is to find an example where the theory should apply, but fails. as an example: swans are birds related to ducks and geese. today, several species of swans are known. for most people in europe, a swan is a large white bird: the only species of swan that occurs in europe is white. several hundred years back, people in europe therefore thought that all swans were white. in 1697, the dutch explorer willem de vlamingh found black birds that looked like swans, during an expedition on the shores of swan river, in australia. later, it turned out these birds were indeed swans. with his discovery, de vlamingh had falsified the theory that all swans were white.