We come to the watermelon field during the harvest season, and the ground is covered with many watermelons. The melon farmer brings a handful of melons and says that they are all ripe melons, and then points at a few melons in the ground and says that these are not ripe, and they would take a few more days to grow up. Based on this information, can we build a model to determine which melons in the field are ripe for picking? For sure, we can use the ripe and unripe watermelons told by the farmers as positive and negative samples to train a classifier. However, is it too few to use only a handful of melons as training samples? Can we use all the watermelons in the field as well?