Often the changes in the two correlated quantities are both the result of a third factor. The odd results above are easily explained in this way. Children with bigger feet spell better because they're older, their greater age bringing about bigger feet and, not quite so certainly, better spelling. Age is a factor in the next example as well since those couples who are older are less likely to divorce and more likely to die than are those from counties with younger demographic profiles. And those nations that add fluoride to their water are generally wealthier and more health-conscious, and thus a greater percentage of their citizens live long enough to develop cancer, which is, to a large extent, a disease of old age.
Researchers call the phenomenon feline high-rise syndrome.
But the sidebar about how they managed to survive the falls was what interested me the most. Most of the cats landed on concrete. Even more surprising, the longer the fall, the greater the chance of survival. Only one of 22 cats that plunged from above 7 stories died, and there was only one fracture among the 13 that fell more than 9 stories. This point, terminal velocity, is reached relatively quickly in the case of cats.