 A drop in the level of mercury in a barometer is frequently followed by a storm. Call these events B and S, respectively. Since storms in general are not so frequent, these events are probabilistically correlated: \(p(B \cap S) &gt; p(B)p(S)\). The behavior of one barometer doesn’t affect the weather, so B is not a cause of S; rather, B and S have a common cause: a drop in atmospheric pressure A (see  Figure 2).  A increases the probability of both B and S: 