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89 tists found that people who lived in Colorado (the USA) had low rates of tooth decay. It was discovered that they drank water which contained fluo‑ ride, a naturally occurring element found in rocks and minerals. In 1945 a water fluoridation program was begun in the USA. Ten years later the test results showed that tooth decay in participating children was reduced by 50 to 70 percent. Now most of the US water supply is fluoridated. The frst fluoride toothpastes were developed in the 1950s. The fild of dentistry continued to develop. In 1957, John Borden invent‑ ed the frst high speed electric hand drill. In the 1960s, the frst dental lasers were developed and used for soft tissue procedures. The 1990s marked the beginning of the era of cosmetic dentistry, with the increased popularity of bleaching and dental implants. Dentistry eventually evolved into specialized practices that include end‑ odontics, orthodontics, periodontics, oral and maxillofacial surgery, radiol‑ ogy and pathology, pediatric dentistry, and prosthodontics. 10. Take turns to retell your text in detail to your partners to enable them to answer the corresponding questions. 11. Discuss in groups. • What information did you fin surprising? • Can you share other interesting facts about the history of dentistry with your group mates? Grammar 1 Present Perfect / Past Simple 12. Revise the formation and use of the Present Perfect tense in English. Scan the texts on the history of dentistry and find sentences in the Present Perfect. Explain why this tense is used there. 13. Work in pairs. Make short dialogues according to the model. Model: – I’ve already translated the text. – Have you really translated the text? – Yes, I have. And you? – No, I haven’t translated the text yet. 1. I’ve already passed the exam. 2. I’ve already read an article on dental caries. 3. I’ve already done my chemistry homework. 4. I’ve already seen this flm.