Green vegetables contain chlorophyll, when chlorophyll is cooked for too long it turns to a grey colour or even brown. To prevent this, add salt to your water when boiling your greens. The salt provides a "barrier" against discolouration, and keeps the chlorophyll green. Always bring the water to a boil before you add the vegetables. This will ensure the pores on the vegetables will be quickly blocked, and prevent vitamins and minerals from leaching out into the water. Cook the vegetables until they are tender. You do not want to overcook or undercook them. It is also important to stop the cooking process as soon as the vegetable is cooked perfectly. The best way to do this is to transfer your green in a bowl of very cold water and some ice. This will also fix the chlorophyll in the greens and keep their colour vibrant. Brussels sprouts are a source of the B-vitamins necessary for cellular energy production, including vitamin B6, thiamine, and folate. Brussels sprouts contain 24 percent of the daily recommended amount of vitamin A, which is great for your eyes and organs. Brussels sprouts also contain manganese, which helps with the metabolism of carbohydrates, amino acids, and cholesterol. Brussels sprouts are an excellent source of vitamin C and vitamin K, with a cup providing more than a day's requirement of these vitamins. Vitamin C is an important water-soluble vitamin that helps to repair tissues and boost immunity. Vitamin K is an important fat-soluble vitamin that plays a role in bone formation and blood clotting.