The buckets are labeled "one tooth fairy" and "two tooth fairies". Two tooth fairies are much less likely than one. The way this works is that you've got some

The buckets are labeled "one tooth fairy" and "two tooth fairies". Two tooth fairies are much less likely than one. The way this works is that you've got some experimental result which appears to violate the laws of physics. After working hard to make sure that it's not experimental error and the measurement means what you think it means, then you consider a change in the laws of physics. This is a tooth fairy. Is your proposed change ruled out by other experiments? Is there a new experiment you can do which will have an unusual result thanks to the tooth fairy? And so on. If, during this process, you find yourself needing a SECOND change in the laws of physics in order to explain something, then that's a second tooth fairy. You don't want that, because that's even less likely to be real then the first one.