简述缩孔形成的原因以及预防措施。

     There are eight kind of tigers in the world. A     
tiger is usually two meters long and two hundreds  
kilos heavy. A mother tiger may have two to four 
babies each  time. About three years late, the   
babies can grow as big like their mother. They     
can stay about twenty years. They like to sleep  
in the day-time and come out to food at night. 
They aren't eat people if they are not too hungry.  
There were three kinds of tigers in China. One      
kind of it lives in the northeast. They are   
the bigger ones in the world, but the number of (数目)
them is very small.