Pear
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.
Imagine standing below a tree full of pears. You take your knife and carefully cut them all down, one by one.
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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.
Imagine a beautiful, ripe pear in your mind. Now gnash your teeth. Mmm. You want to eat a pear right now, don't you?