The village of Ferlinghetti has been famous for it's locally produced mineral water for about 150 years. However, the village also has a problem with adult asthma. Many of the town's adults spend much of their time coughing and wheezing. Locals believe that the mineral water reduces or prevents the asthma and have been drinking more and more of it since the asthma problem was first identified in 1852. The asthma problem has grown slowly but steadily worse, with no interruptions, since the early 1850s. Asthma attacks do not get any worse or any better at any particular time of day or time of year. The leading citizens in the town have identified four possible causes of the asthma. A tin mine in the mountains produces runoff, which mixes with the local drinking water. A perfume factory in the next valley produces fumes, which drift over to Ferlinghetti. Farmers in the nearby plain use a special kind of wheat, which releases pollen that is carried into Ferlinghetti by the prevailing winds. Bats from the local woods fly into the town at night to feed on insects, and leave fur and skin flakes that hang in the air for about an hour after they fly through. The tin mine has been in continuous operation at the same level of production since Roman times. The perfume factory was built in 1843, but was completely shut down for both world war one and world war two. The special wheat was introduced in 1840. The local species of bat was discovered and classified in 1845 when local kids found caves containing both the bats and accumulations of bat droppings that proved to be dozens of feet thick. You are a scientist called in to investigate the situation and discover the cause of the asthma problem. 

The town mayor argues, "it's the tin mine in the mountains, because that runoff water does get into the local drinking water. It must have started running off in 1852 and the amount of water running off must have gotten larger and larger since then."

The chief of police claims that "the perfume factory has to be the cause. There must be something in the perfume fumes that causes asthma, and they must have been using it continuously since 1852."

The town drunk confides in you that "the wheat must be the cause. They've been growing it continuously since at least 1852, so it's perfectly well correlated." 

And the town librarian says "it's the bats! Aaaaaaaaaaagh! They're after me! Keep them away, keep them away! Send for Buffy now!"

Apply careful causal reasoning to the problem. We'll discuss this problem is class.