What episode is treading on thin ice Riruoxi and Grandpa Four writing together?

in episode p>18, grandpa four gave ruoxi a word, "I will walk till the water checks my path", because of the prince's disturbance. If the sunrise likes it very much, she will copy Grandpa Four's words by herself and practice it, feeling as if Grandpa Four is writing with herself. (In fact, it's all Ruoxi's imagination, and they didn't really write with Grandpa Four.)