They are just four inches square and often obscured by the dirt and dust of urban life. But the "stumble stones" implanted into the cities and towns of Germany punch far above their weight in terms of human emotion. With thousands of them across the country and Europe, these tiny brass tablets of information about the targets of the Nazi Holocaust have become the most potent reminders that the victims were humans and not abstract historial statistics. Artist Gunter Demnig's "stumble stones" tell the casual walker that in the house he or she is standing outside once lived people who were rounded up and taken away to be murdered because of their ethnicity, religion, politics or sexual persuasion.