In English, people say dogs are man's best friends. Dogs and humans have been close for thousands of years. Scientists believe the bond is very deep. They found that dogs cry with joy when they see their owner after being away a long time. The scientists studied 22 dogs. They looked at how the dogs reacted when they saw their owners again. When the dogs saw their owners, they cried. When they saw other people they knew, they didn't cry.

The researchers put paper under the dogs' eyes to see if the dogs cried. When dogs saw their owners after five hours of being apart, they cried. A researcher said he had never heard of animals crying tears of joy. He said dog's tears might deepen their relationship with humans. He thinks dogs' tears make humans love their dogs more. He said: "It's possible that dogs who show tears…during human interaction would be cared for more."