The sphenoid sinus is one of the four pairs of paranasal sinuses located within the skull. It is situated behind the nose and between the eyes, specifically in the sphenoid bone. This sinus is shaped like a butterfly and plays a role in reducing the weight of the skull, as well as contributing to voice resonance. The sphenoid sinus is lined with mucous membranes that help trap and clear out pathogens and debris. It drains into the nasal cavity through an opening called the sphenoethmoidal recess. Inflammation or infection of this sinus can lead to conditions such as sinusitis, causing symptoms like headache and nasal congestion.