Watermelons will not ripen after they are picked, so it is important to select a ripe watermelon at the market. Ripe watermelons have a yellow or cream-yellow "ground spot," the place the melon rested on the soil. If this spot is green or white, the watermelon is probably not ripe. If you thump a ripe Watermelon, it will sound like a dull thud rather than a tighter ringing or hollow sound. The rind should be relatively smooth and neither overly shiny nor overly dull. As with all fruits, I recommend selecting locally grown, organic watermelon whenever possible.