### 第1段

1 .<-NARRATOR:-> Now listen to part of a lecture from a biology class.

### 第2段

1 .<-MALE PROFESSOR:-> All right, so I-I've got a good example of this.

2 .There's a bird, a species of crow, that lives near the water,

3 .and it feeds on a type of shellfish that has a hard outer shell.

4 .In order to eat this shellfish,

5 .the bird has to crack open its hard shell.

6 .So when this bird feeds,

7 .what it does is it dives down out of the air into shallow water,

8 .grabs a shellfish in its mouth, then carries the shellfish up in the air....

9 .It then drops the shellfish

10 .lets it fall onto the rocks below.

11 .When the shellfish hits the rocks, its shell cracks and splits open,

12 .and the bird can eat it.

### 第3段

1 .Now this bird, this crow, doesn't just swoop down, grab the first shellfish it sees, then fly up to any height and let it fall.

2 .Instead, it does two things:

3 .first, it carefully selects only the biggest shellfish;

4 .that means it's going to get the biggest possible meal for its efforts. and drops it from there.

5 .If the bird dropped the shellfish from a lower height,

6 .it would have to pick it up and drop it too many times in order to break the shell.

7 .On the other hand, if the bird carried the shellfish up to a higher altitude,

8 .an altitude higher than is necessary to crack the shell,

9 .it'd be wasting energy.

10 .So this bird expends just the right amount of energy-no more, no less-

11 .that it needs to obtain just the right kind of food.