premise: keeping warm is the opposite of becoming cold sent2: thick fur is a kind of fur sent3: fur is a part of skin for keeping warm sent4: if the environmental temperature is too cold then an animal in that environment may become cold sent5: when fur grows it becomes thicker sent6: an animal is a kind of living thing sent7: winter has cold temperatures sent8: if a living thing becomes too cold then that living thing will die.
hypothesis: some animals' fur grows thicker for staying warm in the winter