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In 2004 economics and public health professor Paul Gertler was invited by the Mexican government to study a program called Piso Firme, or "firm floor," a deceptively simple public health initiative in which families were given an average of $150 worth of wet cement for domestic flooring. To measure the efficacy of the program, Gertler and his colleagues traveled to Torreón, in Coahuila, where Piso Firme had been implemented, and neighboring Gómez Palacio/Lerdo, in Durango, where it had not.

Gertler was surprised by what he found. The rates of improved health observed in Torreón, he says, were "just astronomical." Children living in homes with cement floors not only showed significantly lower incidences of diarrhea and parasitic infestations (49 percent and 78 percent reductions, respectively), they also scored 36 to 96 percent higher on cognitive tests. What's the connection? Parasites, Gertler explains, "love to eat micronutrients. And anemia [a shortage of red blood cells caused by a deficiency in micronutrients] is one of the main causes of delayed cognitive development."

Gertler stresses that Piso Firme's success can't be chalked up entirely to cement floors; both communities were also well positioned to benefit from the project. "They had municipal water, they had sanitation. And so this was one of the last vectors by which parasites could infest kids," explains Gertler. It's a reminder, he says, that public health projects need to be approached with the entire living environment in mind. The impact of a new water treatment system, for example, could be undermined by neglecting to replace dirt floors—and vice versa.

The Mexican government is now busy implementing the program nationwide.

In 1971 Maria Gõmez Pereira of Belmez, Spain, noticed the image of a human face on her kitchen floor. A neighbor replaced the floor, but the face returned. Investigators removed the cement floor and excavated headless skeletons and piles of human bones. A new floor was laid, but a male face appeared, then a female face, followed by more faces. Over time, the faces seemed to move, change expressions, and age.