Can someone explain this

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http://www.youtube.com/watch?v=DD2BD3z7gPo if you watched the video above you will see an adult cat still drinking milk from his mother, I think his mother gave birth to a new batch of kittens and this big guy of a cat is now stealing milk that is supposed to be for his younger siblings. I just hope the mother cat wont get wounded cos her over aged son already have sharp fangs. Why do you think some adult cats still drink milk from their mothers?