Just google his name and "rivets". Looks like it's actually a joking remark Card makes himself to express what it's like to dabble in both sci-fi and fantasy (Card is successful with both).

    Card maintains that switching genres wasn't a difficult 
    thing for him. He believes that, ultimately, the 
    difference between the two genres is largely on the 
    surface. "Half joking, I was writing to Ben (Bova) about  
    this very subject, and I said, look, fantasy has trees, 
    and science fiction has rivets. That's it, that's all  
    the difference there is, the difference of feel,
    perception.