Drop hundreds of 'lost' letters and you can map a city's hidden attitudes
Stanley Milgram seeded neighborhoods with stamped, addressed envelopes left looking accidentally dropped, then simply counted how many strangers mailed them onward. Letters addressed to a Communist or Nazi-sounding organization got mailed far less often than ones addressed to medical research, quietly revealing an area's politics without ever asking a direct question. Nobody involved knew they were part of an experiment.