Atoms are all around us. They are the bricks of which everything is made. Many millions of atoms are contained in just one grain of salt，but despite their small size they are very important. The way an everyday object behaves depends on what kinds of atoms are in it and how they act.
For instance，you know that most solid objects melt if they get hot enough. Why is this？ It is the effect of the heat on the object's atoms. All atoms move constantly. When they are hot，they move faster.
Usually the atoms in an object hold together and give the object its shape. But if the object grows hot，its atoms move so fast that they break the force that usually holds them together. They move out of their usual places so that the object loses its shape. Then we say that the object is melting.
11、One grain of salt contains （ B ） .
A、a few thousand atoms
B、many millions of atoms
C、several heated atoms
D、one million atoms
12、The way an object behaves depends on the （ D ）。
A、kinds of atoms in it
B、number of atoms in it
C、way its atoms behave
D、both A and C
13、Heating an object will affect（ D ）。
A、the movements of its atoms
B、the speed of its atoms
C、the shape of its atoms
D、both A and B
14、An object holds its shape because its atoms （ A ）。
A、usually hold together
B、move very fast
C、are very hot
D、are not moving
15、When its atoms leave their usual places，we say that an object is（ D ）。
A、shrinking
B、changing places
C、expanding
D、melting
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