Watermelon

The leaves around the watermelon has turned yellowish... wondering if it is a signal for me to harvest. When I knocked on it, it does have a hollow sound...... I planted 2 watermelon plants in this plot so I am not sure which plant it belongs to. I hope for at least 1 watermelon for 1 plant! I saw another watermelon fruitlet that seems to grow a little bigger today.. will observe the next few days if it continues to grow...