thick fur is a kind of fur & fur is a part of skin for keeping warm ->  thick fur can be used for keeping warm  an animal is a kind of living thing & if a living thing becomes too cold then that living thing will die ->  if an animal becomes too cold then that animal will die  if an animal becomes too cold then that animal will die & keeping warm is the opposite of becoming cold ->  an animal requires warmth for survival  an animal requires warmth for survival & if the environmental temperature is too cold then an animal in that environment may become cold ->  an animal requires warmth for survival in cold temperatures  thick fur can be used for keeping warm & an animal requires warmth for survival in cold temperatures ->  thick fur is used by animals for keeping warm in cold temperatures  thick fur is used by animals for keeping warm in cold temperatures & winter has cold temperatures ->  thick fur is used by animals for keeping warm in winter  thick fur is used by animals for keeping warm in winter & when fur grows it becomes thicker -> some animals' fur grows thicker for staying warm in the winter