Possible Clues:

• Curve on liquid surface
• Curved surface of liquid, caused by surface tension
• Fibrous crescent-shaped cartilage in a joint
• Disk of cartilage in the knee
• Disk of cartilage at a joint
• Curve on fluid surface in a tube
• Liquid surface -- one is fouled up with scum, ring's left
• Curved surface on the top of a liquid
• Lens with a crescent-shaped section