The grain you cut off a tree with a knife is a pear. I don't know if you've ever seen Asian pears, but they're pretty big and sturdy. You can't just pick them off of the tree, you need the help of a knife. That's why this is a pear, and not an apple or peach or something else.
As you cut the pears down, you can't help but gnash (なし) your teeth. You want to eat these pears right now. You gnash and grind and wait though, because you want to have them all down before you start the feast.