In the crystal clear river, we can clearly observe the activities of fish. I saw them, suddenly surfaced for food, and suddenly dive to the bottom to play; Or swim in the water, or stop somewhere in the water and rest motionless. Fish is really free in the water, floating freely. Fish can swim freely in the water, which has a lot to do with its special organ-swim bladder. There are many kinds of fish, such as crucian carp and carp. When we cut open its abdomen, we can see a slender white airbag under its back. This airbag is the swim bladder.

The swim bladder is full of air, which is diffused from the liquid of the fish. The air volume in the swim bladder can be adjusted freely, thus changing its volume. When the swimming fish rises, it will fill the air bag with air. At this time, the volume of fish increases, so does the volume of boiled water, while the weight of fish remains unchanged. At this time, the upward buoyancy of the fish is greater than the weight of the fish, and the fish floats. When the swimming fish descends, it will reduce the air in the airbag, the volume of the fish will decrease, and the volume of the boiled water will also decrease. The buoyancy is less than the gravity of the fish, and the swimming fish will descend. When the air in the air bag is adjusted to an appropriate amount to make the weight of the fish equal to its buoyancy, then the fish can stop somewhere in the water to rest. Fish can rise and fall at will, which is not magic.

Submarines are like fish in the sea. It turns out that there are several pontoons made of steel plates in the submarine, which have the same function as the swim bladder. When the submarine is sailing on the water, several buoys are empty. When a submarine needs to dive into the water, people will pump out the air in the buoy and let the seawater flow in. The more seawater flows into the buoy, the greater the weight of the submarine. When the weight of a submarine exceeds the buoyancy given by seawater, the submarine will dive into the bottom of the sea. Of course, the more seawater enters the buoy, the greater the acceleration of the submarine entering the water. It is not difficult to make the submarine float again. As long as the air pump is started, compressed air is pumped into the buoy to force the seawater in the buoy to be discharged from the buoy, the submarine will lose weight. When the weight is less than buoyancy, the submarine will float again. It can be seen that the secret of submarine ups and downs is nothing more than using buoyancy.