To qualify as the world's tallest man, Sultan Kosan had to be measured by Guinness three times in a ten hour period.
Sultan Kosen in New York City
Guinness officials say that your height can vary, if every so slightly, over the coarse of a day. A person tends to be tallest in the morning, and then gravity does its work.
In 2009, Kosen became the first person in two decades to be measured at over 8 feet.