Well, if you want to get scientific about it, when two people have a kid, parts of what make them who they are go to that child, and the kid becomes a part of the parents. Basically, they have similar DNA.
A non scientific description of that would be they all have similar personalities because the mom and dad passed on some of them to the child. The child would be most alike to their parents by the subconscious actions they take, for example: how they take a shower. For kids who have step parents (me being one), if they grew up with that parent, they eventually adopt things the parent find interests in. Basically, the answer is in the subconscious actions they take and their personalities. <span />
The answer is skin. The skin is also the largest organ in the body and has the most contact with the outside environment. It is estimated that there are approximately 205 identified genera of bacteria alone on the skin. This normal flora of the skin play significant roles in the body such as augmenting immunity.