Between 1907 and 1915 Dr. Black and Dr. McKay, a practicing dentist in Colorado, had a running discussion concerning mottled teeth, or Colorado Brown Stain. Dr. Black spent the summer of 1909 in Colorado Springs to gather information and make observations. Their work culminated in the publication of Mottled teeth: an endemic developmental imperfection of the enamel of the teeth heretofore unknown in the literature of dentistry. (See: Dental cosmos, 1916; vol. 58, pp. 129-153, 477-484, 627-644, 781-792, 894-904)After Dr. Black's death, McKay continued their studies and discovered that fluoride in drinking water was the causal agent of the imperfection, noting the effect on enamel, but the lack of dental caries in those who had the condition. This was a critical breakthrough in understanding the etiology and prevention of dental caries. This discovery is the foundation for water fluoridation, which is the single most effective public health measure to inhibit tooth decay.