當柳樹遇見河流、當蝌蚪遇見毛毛蟲，愛情的火花就此綻放; 毛毛蟲就像蝌蚪眼中的美麗彩虹，而蝌蚪宛如閃亮的黑珍珠照耀毛蟲的內心。一天天過去，相愛的誓言始終未曾改變，彼此允諾與相守;。當毛毛蟲發現蝌蚪長出兩隻腳、身體有了巨大改變之後，曾經深信不移的諾言已讓它心碎，因為一切都不一樣了….。光陰似箭歲月如梭，數年之後的柳樹旁飛著一隻美麗的蝴蝶，她心底依舊念念不忘過去深愛的蝌蚪，正當她詢問河邊的青蛙時，卻突然間被他一口吞了進去…。繪本故事中較少有此類型的內容，透過上下開頁的方式引喻著柳樹與河流的呼應，故事的結局讓人感傷與落寞，然而這就現實的人生，沒有任何事物與情感是永恆不變的，所謂地永遠只能存留於內心留為回憶罷了。
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"Where the willow meets the water, a tadpole met a caterpillar. They gazed into each other's tiny eyes and fell in love. 'Promise me that you will never change', the caterpillar says." And foolishly, the tadpole promises. But as the seasons pass and he matures, his legs grow, and then his arms—and what happens to his beautiful rainbow friend? As he sits on his lily pad, digesting the butterfly he has just eaten, he little realizes that now he will never know!
