The term 'WATERMELON SOURCE' can be solved by the four letter word 'VINE'. A watermelon is a large, round fruit with a hard green rind and sweet, juicy red flesh. It is a member of the cucurbitaceae family, which includes other fruits such as squash and cucumbers. Watermelons are typically grown on vines, which provide the necessary support for the heavy fruit. Vines are also important for providing the water and nutrients that the watermelon needs to grow.