Before I answer, let me give you a little biology background in genetics: All living organisms have these crazy things called genomes. Genomes contain all of the DNA present within an organism. DNA is genetic material that contains all the information necessary for the structure and function of an organism.
Let me share an interesting little example for you. If you have a bacteria of species X, you take its genome out, and add in the genome from bacterial species Y, bacteria X will turn into bacteria Y. That’s right- species X will transform into species Y. If you don’t believe me, ask Craig Venter: http://www.ted.com/talks/craig_venter_unveils_synthetic_life.html
This means that the identity of an organism is defined by the DNA that is present in its genome! You are a human because you have a human genome. I also am a human because I have a human genome. Winnie-the-Poo is a bear because he has a bear genome (Grizzly?). Free Willy is a killer whale because he has a killer whale genome. Babe, Lassie, and Flipper are all… I digress.