You may have witnessed this scene on the road quite often but the answer to why dogs sniff each other's butts is hidden in the chemical communication at the rear end.
To reveal the mystery, researchers from the American Chemical Society have uploaded a video on YouTube that clears the air.
According to the video, it is all about one dog sniffing out key information about the other -- its gender, emotional state, diet and more.
Dogs have a second olfactory system known as the Jacobson's organ.
The canine's nerves direct the chemical information it detects directly to the brain so there is no interference from other odours like those from the poop.
"Think of it kind of like speaking with chemicals," the video narrator says.
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"In fact, this butt-sniffing action is just one of many examples of chemical communication in the animal kingdom," the video narrator adds.
A dog's sense of smell is very acute - 10,000 to 100,000 times stronger than humans, the video states.