INPUT ARTICLE: Article: Figure out the boundaries of the region for which you want to know the population density. Think about why you want this number. Perhaps you want to find the population density of your country, your city, or your neighborhood. You will need to find the total area of this place: usually in feet, miles, meters, or kilometers.  Odds are that someone else has already measured and surveyed this area. Look for census data, read an encyclopedia, or run a web search. Find out whether the area has any set boundaries. If not, you'll need to define them yourself. A neighborhood, for instance, may not be listed in the census, so you'll need to draw the borders yourself. Short of counting the population yourself, you'll need to find an up-to-date record of how many people live in this area. Start by running a web search for the population of the place – say, the city of Houston, Texas. Look for the most recent census data for a relatively accurate number. If you are looking for the population of a country, the CIA World Factbook is a good source. If you are calculating the population density for an area that has not already been recorded, you may need to count the population yourself. This might include a colloquially-defined urban neighborhood, for instance, or a population of kangaroos in a section of the Australian Outback. Try to get as accurate a number as possible. If you plan to compare one area against another, make sure that all of your numbers use a common unit of measurement. For instance, if one county is listed in square miles and another is listed in square km, you will need to convert the area of both countries into either square miles or square km. For easy Imperial-to-metric conversions, visit http://www.metric-conversions.org.

SUMMARY: Define the area. Determine the population. Balance your data.

In one sentence, describe what the following article is about: To make your own pH indicator, you will need half a head of red cabbage, a knife, water, a stove, and a medium saucepan. Red cabbage contains a chemical called anthocyanin that changes color based on acidity. By boiling the cabbage, you can make a pH indicator from this chemical and test the acidity of your rain. Adult supervision is recommended for this because you will need to handle a hot stove and a sharp knife. . You want about 2-3 cups of red cabbage cut into small pieces. If you are young, have an adult help you with this step to avoid potential injury. The exact amount of cabbage is not important, so just cut up half of the head of cabbage. Place the chopped cabbage into the saucepan and add enough water to cover the cabbage. Put the saucepan on the stove and bring the water to a full boil. Once the solution boils, turn the heat off and let the water cool to room temperature.  Have a parent help with this step to avoid injury when using the stove and working with boiling water. You should notice that the water is now purple. Pour the cabbage and water mixture through a strainer into a bowl. The purple water (the indicator) is currently neutral because the water from your tap is neutral. In the presence of an acid, the indicator turns pink. In the presence of a base, the indicator turns blue. You can refrigerate the solution until you need it. To make sure the indicator is working properly, you can test it using baking soda and vinegar. Put 1 teaspoon (~5 mL) of the cabbage water into two small, clear glasses. Add a teaspoon of vinegar to one glass and a teaspoon of baking soda to the other.  Vinegar is acidic, so the water will turn pink. Baking soda is basic, so the water will turn blue. Collect some rainwater in a clean, plastic container. Pour 1 teaspoon of cabbage water into a small, clean glass. Add a teaspoon (~5 mL) of the collected rainwater to the cabbage water. The water should turn pink because all rain is slightly acidic. The deeper the pink, the higher the acidity in your area. This method will not tell you the exact pH of the rain, but will give you an idea of how acidic it is. If you are traveling, you can compare rain from different areas such as different cities, state or province, or even countries and see if the deepness of the pink changes.
Summary: Gather the necessary materials. Chop up some cabbage Boil the cabbage. Strain out the cabbage and save the water. Test the indicator. Test the acidity of your rain.

INPUT ARTICLE: Article: Goldfish, Rosy Barbs, and Guppies are excellent starter fishes. Before you buy your tank and other supplies, determine what kinds of fish and how many fish you will have in the tank. This way, you can buy an appropriate sized tank and the appropriate gravel or substrate. It is recommended that you buy a large tank instead of a small tank. Larger tanks are easier to clean, and you can add more fish to larger tanks. Avoid common myths that tell you it is acceptable to keep fish in bowls. Larger tanks also allow your fish to grow while still being comfortable. Keeping fish in an overcrowded tank is the fastest way to reduce water quality and introduce toxins and diseases to the water.  It is recommended that you start with a fish tank that can hold between 20 and 55 gallons. For smaller fish, use a tank that can hold around 10 gallons. It is also better to buy a wide tank instead of a tall tank. Wide tanks have more surface area, and thus, allow more oxygen to enter the water and more waste gasses to escape. Gravel, or substrate, comes in various sizes, colors, and forms. You can choose from gravel, sand, or crushed coral. Gravel is the most common form of substrate and comes in small to large sizes. It is recommended that you buy gravel that is medium in size.  Do not buy gravel with sharp edges. The sharp edges can harm the fish's scales. Fish that like to pick up the gravel or dig into the gravel are especially at risk. Sand substrate is great for fish with delicate barbells, such as the Corydoras catfish. However, it is very difficult to clean and can get stuck in your filter. If you have a picky eater, like a puffer fish, it might swallow the substrate. Crushed coral gravel is great for fish that prefer a higher pH, such as African cichlids, as this reflects on their natural habitat. There are three different types of filtration systems: external, internal, and under gravel. However, whichever you choose, make sure it is high-quality. Internal filters are great for small tanks.  Under gravel filters move the water through the gravel, which induces biological filtration to occur. They pull water in and they release it out of the spout. Examples of high-quality filters are the Fluval External filter, Penn Plax Cascade canister, and the EHEIM Classic External canister. There are a variety of decorations you can choose from. There are aquarium backdrops, rocks, driftwood, plants (both real and fake), and furniture or decoration caves, tunnels, and bridges. Make sure to purchase a variety of decorations. A starter kit tank though comes with a rather childish decor, as it is marketed toward children.  Plants and furniture allow your fish to hide out, feel safe, and relieve stress. However, for more greedy, herbivorous fish, make sure to get more sturdy, nip-proof plants. Aquarium backdrops are great for hiding cables and equipment behind the tank. They also create an illusion of depth for the fish. Make sure the decorations you buy are safe and non-toxic sharp objects are harmful to fish, especially to the Eye goldfish and the Black Moor fish.

SUMMARY:
Determine what kind of fish you want. Buy a tank. Select the appropriate gravel. Choose a high-quality filter. Pick your decorations.