The greyhound is the world's fastest dog. It can run at speeds of up to 40 miles per hour--that's faster than the average speed in the Tour de France bicycle race One of the oldest dog breeds, greyhounds were raised as hunting dogs hundreds of years ago in Europe. Their speed and agility helped them catch their prey. These are some of the surprising facts kids will discover as they learn all about these lightning-fast canines. Eye-popping photos...