What causes sinus pressure? I mean physically, what's really going on in there?

Snot is good for protecting you from all sorts of trouble-makers, like dirt and germs and allergens. But sometimes your immune-system over-reacts and produces too much. Then the linings of your sinuses get inflamed and swell. So the pressure is from a) too much snot and b) swelling (which is probably the worse of the two problems). That's why docs prescribe those nasal inhalers like Flonase--they're simply steroids that bring the swelling down in your sinus passages.

I'm no expert but....

Sinuses are hollow cavities that open into the nose. The lining of the sinus is a mucous membrane, so it tends to produce mucous. If the opening gets blocked - either through mucous drying up or thickening, or through swelling of the tissues, the continued production of mucous could build up and cause pressure.

Excuse me I think I'm going to take a sudafed to make sure it doesn't happen today...

Mucus, mostly coming from your eyes.

the little smurf type creatures that live and operate inside your nose go on holiday, and they make the rectum smurfs look after the house, but the rectum smurfs throw massive partys and make a huge mess, clogging up the sinuses.

damn those rectum smurfs....

www.webmd.com ... learn to love it :)