How to Make Delicious Acorn Coffee (2016)

Meet Riku, a boy who loves to collect acorns. One day, a girl named Koaki becomes interested in him. She brings him acorns every day to try and garner attention from him, but he always throws them away. Something about them must be different from the ones he collects, but Koaki can't figure out what that difference is. How is he choosing which acorns he keeps, and why?