Answer: Amphibians are animals between fish and reptiles, and have dual lives. Its characteristics are: first, its skin is thin and moist. It consists of epidermis and dermis, including most glands, and there are many blood vessels under the skin. Second, the amphibian lung is saccular, and its interior is divided into tiny terminal air chambers. In addition, the heart of amphibians is divided into two ears and a ventricle. Blood circulation is divided into large circulation and small circulation.

▲. Where is the secret of bird flight?

A: Birds eat a lot because their bodies are spindle-shaped and their bone marrow is full of air. Digestion is also particularly fast, which makes the bird's physical strength particularly strong. In addition, the developed nervous system, strong eyesight and relatively light weight of birds are also important factors for birds to be good at flying.

▲. Why can a rooster announce the dawn?

Answer: Because there is a "biological clock" in the cock's brain. According to scientific research, there is an endocrine organ called pineal gland between the cock's brain and cerebellum, where a "biological clock" grows. Because the pineal gland can secrete a kind of black tensin at night, the rooster can remember the law of light and shade and sing periodically.

▲. Is the donkey really stupid?

Answer: In daily life, donkeys have become people's good helpers, even better than horses and other animals in some aspects. On steep places and steep slopes, horses break their legs when walking, while donkeys can run freely. When there is no road, the donkey can choose a convenient road accurately. It can also do some heavy physical work. Therefore, the donkey is not stupid.

▲. Why do mice like to bite people?

Answer: Because the dental pulp of mouse incisors not only exists for life, but also grows for life. In order to prevent the front teeth from getting too out of the way, it is necessary to grind them off frequently by biting hard objects so as not to grow too long and affect eating.

▲. Why is the elephant's nose so long?

Answer: The elephant's physique is getting bigger and bigger with the change of the environment and its adaptation to the environment, so the distance between its mouth and the grass growing on the ground is getting bigger and bigger, and it is difficult to eat food on the ground. Coupled with the lack of flexibility, it is very inconvenient to move. In order to make up for these shortcomings, the elephant's nose slowly developed into what it is today.

▲. How do ants find food?

Answer: When an ant finds some food, it will immediately return to its nest. When it walks, it secretes a chemical substance and leaves traces on the road. When it returns to its nest, it exchanges information, and ants move food through the smell of this chemical.

▲. Is it true that only female mosquitoes bite people?

A: Yes. Because the purpose of female mosquitoes feeding blood is to supplement nutrition and make eggs mature. Most kinds of mosquitoes can't lay eggs without sucking blood. Male mosquitoes can't bite people because their mouthparts are underdeveloped, and they can only rely on nectar and plant juice to maintain their lives.

▲. Why is the place bitten by mosquitoes itchy and swollen?

Answer: Because human blood is easy to coagulate, mosquitoes will inject saliva into the wound after inserting their mouths into human skin. This saliva can prevent blood from clotting, so that people's blood can be sucked into their stomachs. Mosquito's saliva contains irritating substances, and people respond to this stimulus, so the wound is itchy and swollen.

▲. Why do dogs often wag their tails?

Many animals use their tails to express their feelings. The dog wagged its tail to show that it was happy. Because they can't speak and can't do so many complicated movements, they can only express happiness with the movements of their tails.

▲. Why do dogs bark at strangers?

Answer: Because animals have their own territory, when dogs are wild animals, they all live in groups. Whenever someone or other animals approach, the dog who finds this situation will bark and inform his companion. Now people keep dogs at home, and dogs regard your home as their own, so they bark at strangers.

▲. Why do horses' ears always shake?

A: Horses are as emotional as people. They can only express their feelings through the movement of their ears. Therefore, the horse's ears are always shaking.

▲. Why do pigs love to sleep late?

Answer: Because there is a substance (called endorphin) in the pig's brain, which has anesthetic effect, and because pigs are particularly afraid of heat, they will not move around, so pigs especially like to sleep.

▲. Why are cats not afraid of heat?

A: When the weather is very hot in summer, the ground temperature is relatively low. Cats use this situation to exchange heat between their bodies and the ground to regulate their body temperature. Therefore, cats are not afraid of heat.

▲. Why do ducks sway when they walk?

Answer: Because ducks mainly live in water, their feet lean back and their bodies are heavy first and then light. In order to keep the balance when walking, it has to raise its head and straighten its chest, so it will shake when walking.

▲. Why is the fish's body cold?

Answer: Because fish is a cold-blooded animal, its body temperature is usually similar to that of water, so even in summer, the fish's body is cold.

▲. Why is the belly of the fish facing the sky when it dies?

Answer: Because the upper part of the fish's body is heavier than the lower part, after the fish dies, its back will face down and its abdomen will face the sky.

▲. Why can't eels die without water?

Answer: Because eels are amphibians, they breathe through their gills in water and their mouths on land. Therefore, eels can live out of water for some time.

▲. Why do swallows fly very low before it rains?

A: Because before the rain, the air pressure in the sky was relatively low, and then all the small flying insects flew very low, so the swallows had to fly very low in order to prey on these flying insects.

▲. Why don't tigers eat grass?

A: Because the digestive organs of tigers can't digest grass, and tigers like to eat lively animals, so tigers don't eat grass.