The cat opens its mouth and sticks out its tongue.
If you notice your cat with its mouth open and tongue sticking out, it might be because it has just engaged in strenuous exercise, is too hot, and needs to release excess heat. It could also be stress caused by tension or fright; in this case, you can try to soothe your cat. However, if this is accompanied by heavy panting or rapid breathing, it may indicate an underlying medical condition, and you should seek veterinary attention immediately. 
1. Facilitates heat dissipation
When the weather is too hot, or after strenuous exercise, a cat's sweat glands cannot expel a large amount of heat, and its heart rate will increase. Therefore, the cat will open its mouth and stick out its tongue to expel heat and cool down.
2. Stress response
When a cat is stressed or frightened, it experiences stress, leading to a rapid heartbeat and shortness of breath. It may also need to open its mouth and stick out its tongue to regulate its breathing. At this time, the owner can gently touch the cat to help it calm down.
3. Suffering from an illness
If a cat is panting heavily or breathing rapidly while opening its mouth and sticking out its tongue, it may have a respiratory illness, bronchitis, cardiovascular disease, oral disease, or asthma. In this case, the owner should take the cat to the vet as soon as possible for appropriate treatment.
