We all have cognitive biases. Most often, these biases stick us with nothing more than bad cellphone service or a little gambling debt. Occasionally, though, they cause us to accuse our neighbors of espionage.

The reason why is called the Clustering Illusion. Randomness isn't obvious, and there's a way to demonstrate that. An interesting exercise, given by many math teachers, involves two groups of people making two records of the results of a series of coin flips. One group actually flips a coin fifty times and records the results, and the second group fakes a record. It's remarkably rare for the false record to look anything like a genuine one. The false record will have runs of two or three tails, before the fakers get nervous and switch back to heads. They think that clusters of similar outcomes only happen when the data isn't random. The real record will have long runs, often of seven or eight tails in a row. Although if you look at those clusters, the coin looks weighted, it's just the result of random chance. Clustering of certain outcomes is just another aspect of randomness.