Why do dogs get stuck while mateing?

there is a mass of flesh at the base of the dogs penis, during copulation the dog inserts its penis into the bitches vagina. once the mass of flesh enters the vagina it starts to swell, this is called the knot. when the knot swells the penis becomes stuck, this is called the tie and can last up to 30 minutes, in some Dog Breeds the knot can swell to the size of a small grapefruit. this ensures the penis stays inside the bitch long enough for successful reproduction.

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