There are several reasons why a cat might lie down and expose its belly:
1. Because it trusts its owner completely and is willing to expose its weaknesses.
2. It's also possible that it's trying to get its owner to play with it.
3. Heat dissipation. Because cats' sweat glands are not well-developed and their heat dissipation efficiency is very low, if the weather is hot and the indoor temperature is high, they will turn over to dissipate heat.
4. If your cat is very itchy and needs to roll over or turn its belly to relieve the itch, be aware that it might be infected with external parasites. Rolling over is actually a way for the cat to cry for help, and you should observe its skin condition carefully.
5. Seeking attention. The owner may have neglected the cat for a long time, so the cat uses this method to attract your attention and hopes you will spend more time with it.
6. Begging for treats. If you haven't given your cat a treat in a long time, it might roll over onto its back to make you happy, and then reward it with a treat.
7. Ask to have its belly patted. Because if the owner knows the right time and pressure, patting a cat's belly can actually make them feel very comfortable. 
Explanation and introduction of common cat behaviors:
1. Cats like to drink water from their owner's cup because they think the water in the cup is clean and fresh.
2. If a cat licks the wall, it may be due to a deficiency of a certain element in its body. In this case, the owner must stop the cat immediately and supplement it with an appropriate amount of trace elements.
3. Cats love to step on keyboards because they are attracted by the movement of fingers on the keyboard, and the sound of typing also amuses them.
4. Many cats like to crawl into cardboard boxes because the small space inside gives them a greater sense of security.
