Simpson's paradox is amazing, it's worth reading all the examples in the article and really understanding it.
Another good source is Martin Gardner's "Aha! Gotcha" book. He presents the paradox as a woman trying to find eligible bachelors at a party. In room 1 her odds are better if she goes for guys with mustaches. In room 2 her odds are also better if she goes for guys with mustaches. But when everyone goes into one room, her odds are better for guys without mustaches. It's incredible.
Another good source is Martin Gardner's "Aha! Gotcha" book. He presents the paradox as a woman trying to find eligible bachelors at a party. In room 1 her odds are better if she goes for guys with mustaches. In room 2 her odds are also better if she goes for guys with mustaches. But when everyone goes into one room, her odds are better for guys without mustaches. It's incredible.