Hibernating is a special l-o-o-o-ng sleep. This ground squirrel has gone to sleep for the whole winter. Other animals do as the ground squirrel does. They go to sleep for most of the winter-they hibernating. Some hibernating animals take only one breath every few minutes. When the weather gets warm again, the heart of the hibernating animal begains to beat faster. 

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Correction:  Student's sentence   Corrected sentence        Grammar explanation       Better 

Hibernating is a special l-o-o-o-ng sleep. T

This sentence is correct.

his ground squirrel has gone to sleep for the whole winter.

This sentence is correct.

Other animals do as the ground squirrel does.

This sentence is correct.

They go to sleep for most of the winter-they hibernating.

This sentence is correct.

Some hibernating animals take only one breath every few minutes.

This sentence is correct.