I think we need something sticky in our noses to stop icky stuff from
getting in our bodies. I remember working in a yard of dirt with a
runny nose and every time I blew it out, my mucus was black. Better
that than my lungs.

It probably has to start out as liquid in order to fit through our
pores or nasal membranes or where ever it comes from, and the air
sometimes dries it and it becomes more solid.

There's more information at
http://www.allergycapital.com.au/Pages/Milk%20Mucus.html