|Charlie sticking his bum in air|
Why do cats show you their bum? There is one situation under which cats do show you their bum. One of these is when we stroke our cat's back especially the lower back. A cat might raise her bottom and present it. This seems rude to us but is actually natural for our cat. It is a throwback to when the cat was a kitten.
The kitten's mother will lick her kitten's bottom to stimulate defecation, something that is important from a health standpoint. Mothers will also lick her kitten on their back and other parts of the body.
When we stroke our cat's back we are acting as a surrogate mother and our cat's instinctive, reflex response is to raise her bottom for her mother to lick it.