OIK: what's my rule?

This week's OIK Tickler is from Neal, who watched closely as I sorted a set of apples.  We had already sorted by color and by size; "what's my rule for sorting this time?"

Pointing, Neal said, "Those are the sticking-up-things, and those are the not-sticking-up-things."

He was right: one group of apples had stems and the other didn't.  What can we make of his general-yet-specific description?  Post your responses in the comments!