First, the stones keep birds from devouring the watermelons. When birds descend on melons with stones on top, they will not be able to stand firm and eat. This will startle the birds and cause them to fly away.

Growers then mark ripe melons. Watermelons can take up to a month to mature after fruiting.

Third, people place stones on watermelons to protect them from sunlight.

Fourth, let the watermelons grow equally. When watermelons reach a particular size, the fruits rise vertically. If the stones are heavy enough, the melons will grow more uniformly.
Fifth, increase the melon quality.