鱼靠什么上浮和下沉？

Most fish have a swim bladder. 大多数鱼都有一个鱼鳔。 The swim bladder is like a small balloon inside the fish. 鱼鳔就像鱼肚子里的小气球。 When the fish wants to go up, the bladder fills with gas. 鱼想往上浮时，鱼鳔就充满气体。 When the fish wants to go down, the gas is released. 鱼想往下沉时，就把气体放掉。 So the fish can float without swimming. 所以鱼不游也能浮在水里。 A dead fish floats because its bladder is still full of gas. 死鱼会浮起来，因为它的鱼鳔还鼓着气。 That is how fish go up and down in the water. 这就是鱼在水里上浮下沉的秘密。