Possible Clues:

• The sun and moon, for two
• Balls, geometrically
• Baseballs and basketballs, but not rugby balls
• Shapes of many Christmas ornaments
• Ball-shaped solids
• Circles' three-dimensional counterparts
• Baseballs and basketballs, but not footballs
• Wasp he researched gathers in fields
• Sheep's wandering around end of farmer's fields
• They look round, quietly present on board