A predator kills and eats other animals as prey （猎物）. Sometimes it also becomes food for other predators. Polar bears are the largest of all the land predators. They can run across snow and swim in icy water for hours without rest. Their favorite food is seal. Not all big animals live on small animals. Hippos have heavy bodies and huge mouths. But they don’t eat animals. They just eat grass.
Some predators use speed to catch their prey. Cheetahs are the fastest animals on land. They can run up to 70 km an hour. Other predators use their senses to find their food. Sharks can see, smell, hear, feel and taste. They can also sense electrical currents（电流）from other animals’ bodies. Some fish try to hide themselves, but with this sixth sense, sharks can always find them. Sharks have smooth bodies. This helps them swim without using lots of energy. They need to save their energy because they never really sleep and never stop hunting. Flying is a useful skill for many predators in the sky. The butcher bird is the most interesting predator. It has a special way of eating its prey. First it kills its prey. Then it hangs it on a fence or in a tree, just like a butcher hangs meat in his shop. Finally, it eats its prey.
Some people hunt and kill more animals than any other predator. They not only hunt for food. They also hunt for fun. They kill sharks for their fins, and tigers for their skins. Those human hunters are the worst predators on Earth.
把划线部分译成汉语。
All big animals live on small animals according to the passage.  (      )
Predators may use _________ or _________ to find their food.
Why do sharks have smooth bodies?
_______________________________________________________________.
How does the butcher bird eat its prey?
_______________________________________________________________.
_______________________________________________________________.