Possible Clues:

• Element used in alloys
• Element no. 39
• Silvery metallic element
• Element #39
• Rare-earth element
• Rare-earth element whose atomic number is 39
• Rare-earth metallic element, atomic no. 39
• Showing unusual maturity, but lacking a certain element
• Element with symbol Y
• Silvery metal, Y, used in radar technology and and to strengthen some alloys