Where do Tieflings come from?

Since Outsiders neither reproduce ***ually or have what we consider to be
life functions (Outsiders do not eat or breathe, etc.), where do
tielflings 
and other half-breeds come?

-- 
It is simply breathtaking to watch the glee and abandon with which
the liberal media and the Angry Left have been attempting to turn
our military victory in Iraq into a second Vietnam quagmire. Too bad 
for them, it's failing.