资料题：The word "nibble"(in the last paragraph)means（　）。

Some of us count sheep to help get to sleep at night.There are millions of them in the world to count.In fact, there are about a thousand million! For about every five people in the world there is at least are sleep.Sheep are among the most useful animals.They give people meat, wool, and leather.In some countries, people drink sheep's milk.Other people use the milk to make cheese.There are many different kinds of sheep.A full-size sheep may weigh from 100 pounds to over 300 pounds.They grow from two to four feet.Their coats of wool may be white, gray, brown,or black.Some sheep have short, fine wool coats.Others have long, rough wool.Some have horns, but others have none.Sheep live well together and follow one another in herds, or flocks.Since these flocks usually stay together on their own, one person called a shepherd, with a sheep dog, can take care of many sheep at one time.The dog will tum wandering sheep back to the group.It will also fight attacks by other animals.Sheep spend most o the day eating grass, leaves, and small plants.If a strange animal or person comes near the flock, the sheep will stop nibbling food and will put up their heads to watch.Then they will run toward each other until they form a tight group.Doing this makes them feel safe.Finally, an alder sheep will lead the flock away to another part of the field.