Q:

What leads to the different types of navels?

Obviously, there are "innies" and "outies" in the human belly button world.  Both are the result of a knot tied in the umbilical cord just after birth.  
But why do some belly buttons become outies while others become innies?  Is it related to how the doctor ties the knot, or is it related to how the knot heals?  

A:

After childbirth, the umbilical cord is either tied (similar to a tourniquet - the cord itself is not tied into a knot) or clamped to cut off the blood supply. It is then cut distal to the clamp/tie, separating the child from the rest of the cord and the attached placenta. The remaining cells die and dessicate, and the stump eventually falls off, forming the umbilicus or belly button. The default result, barring any issues, is an "innie". However, an "outie" may form if the child was born with a tiny umbilical hernia, or there was a small unnoticed infection at the base of the umbilical cord. Either of these could lead to the formation granulation tissue, described colloquially as an "outie". People can also have combination navels, with the indentation of an "innie" but a small amount of protruding granulation tissue inside.

