A fish has a swimming bladder inside it-that is,a sack containing air. If it wants to dive deep down and stay down,it squeezes out some of the air and lets in water.

• 鱼肚里有鱼鳔，也就是盛空气的袋子，如果它要潜下深水不上来时，就吐出一些空气，把水换进去。