Trust and safety. When a cat lies on its back, it means that it feels safe and trusts those around it.
Comfort and relaxation. Sometimes cats may lie on their backs to keep warm from the floor. They can also relax in this position and feel very comfortable at the same time.
Excess weight. Some cats have a large belly, making it difficult for them to lie in positions other than their back.
individual preferences. Cats may have their own favorite poses, too.
Itching. If the kitten or cat cannot reach the itchy area with its teeth and tongue, it begins to rub its back on the ground to relieve this discomfort.
Games. Sometimes lying on your back is part of cat play.
Source: Ferra

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