Fish live in water, but they still need oxygen to survive.
鱼生活在水中，但它们仍然需要氧气来生存。
Instead of lungs, fish have special body parts called gills.
鱼没有肺，而是有一种特殊的器官叫做鳃。
Gills help fish get oxygen from the water.
鳃可以帮助鱼从水中获取氧气。
When a fish opens its mouth, water flows in.
当鱼张开嘴时，水就会流进去。
The water then passes over the gills.
水接着流过鱼的鳃。
Inside the gills, tiny blood vessels take oxygen from the water.
在鳃的内部，微小的血管从水中吸收氧气。
At the same time, the gills release carbon dioxide back into the water.
同时，鳃会把二氧化碳释放回水中。
This is how fish breathe underwater.
这就是鱼在水下呼吸的方式。
Humans do not have gills.
人类没有鳃。
That is why we cannot breathe underwater.
这就是为什么我们不能在水下呼吸。
If people go underwater, they need special equipment like oxygen tanks.
如果人类进入水下，就需要像氧气瓶这样的特殊设备。
Some fish, like sharks, must keep moving to push water over their gills.
一些鱼，比如鲨鱼，必须不停游动，才能让水流过它们的鳃。
Other fish can stay still and still breathe.
而有些鱼即使不动也能呼吸。
Polluted water can make it hard for fish to get oxygen.
污染的水会让鱼更难获取氧气。
That is why clean water is important for fish to survive.
这就是为什么清洁的水对鱼的生存非常重要。
Some animals, like whales and dolphins, live in water but have lungs.
一些动物，比如鲸鱼和海豚，虽然生活在水里，但它们有肺。
They must come to the surface to breathe air.
它们必须浮到水面上呼吸空气。
Fish do not need to come up because they use gills to get oxygen.
而鱼不需要浮出水面，因为它们用鳃来获取氧气。
Now you know that fish breathe with gills, and humans cannot breathe underwater!
现在你知道鱼用鳃呼吸，而人类不能在水下呼吸了！
