Extent
The Grain Mouth King pours grain from his mouth for you and his other subjects. You go to him and pull his finger. When you do this, grain pours from his mouth. But we were greedy. We didn't understand the extent to which we were harvesting his mouth wheat.
Of course, you need to imagine pulling the Grain Mouth King's finger, to some extent. You can't just read what we write for you, without going to the imagination zone. Pull that finger real hard a couple times, and watch an extra large purge of grain pour forth.
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The extent to which we were harvesting grain from his mouth was too great. The Grain Mouth King has run out of grain. He worries he will lose power if he has no grain to offer, so he switches to barfing out tater (てい) tots instead.
Imagine hearing the rumors that the Grain Mouth King ran out of grain, so you go check the extent of his failure yourself. But tater tots taste pretty good too. Imagine eating one of his mouth tater tots and go ahead and comment about it to him.