When potatoes are exposed to light, chlorophyll is formed which provides the green color, but another by-product is also formed: solanine. If enough is eaten it could cause symptoms such as vomiting and diarrhea. Because of the bitter taste of solanine, it is rare for someone to eat enough to actually get sick. If you find small green spots, simply trim them off. If more extensive greening has occurred, throw the potato out. Remember to store potatoes in a cool, dark place.