To unblock a greasy sink, first try using a plunger to dislodge the obstruction. If the plunger does not work, use a sink auger to clear the blockage.  Turn off the sink's water supply, insert the auger into the drain, and turn the handle clockwise to force the end of the auger through the clog. Once the obstruction has been cleared, turn the water supply back on. If the clog remains, you may need to call a plumber.