Male cicadas sing together to attract mates, but this also attracts birds who see them as a tasty meal. Singing together confuses the birds, allowing female cicadas to identify nearby males. Some species of cicadas synchronize their singing to attract more females from a larger area. Cicadas have more than 3000 species and spend most of their lives underground as nymphs. They use an organ called a tymbal to sing, emitting some of the loudest sounds of any insect. Singing in unison does not deter predatory birds, as some have suggested. The Neoptera division of insects, which includes cicadas, also includes other "singing" insects like crickets. Hilly towns may not necessarily have better health than flat towns, as factors such as diet and access to fresh produce can be affected by the terrain. Hilly towns may have less access to cheap supermarkets and other retail areas, and excessive exercise in carrying heavy loads uphill may cause joint wear. However, small hilly towns may attract an older, wealthier population with better cooking knowledge. Popping knuckles does not lead to arthritis, as confirmed by a study comparing the effects of cracking knuckles. Humans are not covered in hair like other primates, but the reason for this is unknown. [Extracted from the article]