纳米技术是指纳米尺度内的科学技术，它是现代科学技术的前沿，纳米是很小的长度单位，

选词填空（10分）

Stay,   far,  carry,  then,    fall,  do,  leave,  down,  push,  high

         A rain drop fell to the ground on the side of a hill. The rain drop ran  1 the hill into a small river. The small river   2  the rain drop to a big river. There the rain drop travelled  3 to the east and mixed itself with salt of the sea. The sun shone brightly. It warmed the water on the surface of the sea. Though the water became warm, some of it turned into water vapor.
So  4  the water of the rain drop. The water vapor  5 the sea and went into the air. It do not carry the salt.    
The cold air  6 the warm air  7 above the ground. The warm air became cool when it went up.  8 the water vapor of the rain drop turned into small drops gathered bigger and bigger drops. At last they became so big that they were too heavy to  9 in the cloud, and they  10 to the ground. So the rain drop would start another trip to the sea.