Stuff happens. The weather forecast says it’s sunny, but you just got drenched. You got a flu shot—but you’re sick in bed with the flu. Your best friend from Boston met your other best friend from San Francisco. Coincidentally. What are the odds? Risk, probability, chance, coincidence—they play a significant role in the way we make decisions about health, education, relationships, and money. But where does this data come from and what does it really mean?