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ImageA sea turtle glides over your head. Tiny blue and yellow fish flash by. A big shark swims right at you. You are walking around on the bottom of the sea. Yet you are perfectly safe and dry. You are visiting an aquarium.


An aquarium is a water-filled tank in which fish swim about. You can keep a small aquarium at home. You can visit a large aquarium in most cities. This kind of aquarium is a building with fish and other water animals in large tanks. A large aquarium may have otters, turtles, dolphins, and other sea animals. Most aquarium tanks also have plants.


ImageColorful tropical fish swim through warm water around a coral reef. The fish are part of the coral reef ecosystem. Beautiful orchids grow on trees that tower above the rain forest floor. The orchids and the trees are part of the rain forest ecosystem. A hawk swoops down on a rabbit hopping through the tall grass of a prairie. The hawk, rabbit, and grass are part of the prairie ecosystem.


An ecosystem is all the living and nonliving things in a certain area. All the plants and animals, even the microorganisms that live in the soil, are living parts of an ecosystem. Air, water, and rocks are nonliving parts of an ecosystem.


ImageDo spiders date? Do sponges have parents? Are there boy plants and girl plants? All these questions have to do with reproduction. Every species (kind) of living thing reproduces. Reproduction is how a species survives.

Sponges reproduce in simple ways. Spiders, plants, and people reproduce in more complicated ways. But there are just two basic ways of reproducing: by asexual or sexual reproduction.


Asexual reproduction means that there are no male and female parents. There is only one parent. All the offspring (new organisms) that develop are just like the parent. They all have the same genes as the parent.


ImageYou might think that the Arctic would be a terrible place to live. For most of the year, there is nothing but ice and cold weather. A polar bear thinks the Arctic is the perfect place to live. That’s because the Arctic is the polar bear’s natural habitat.

You might think that a tropical island is a wonderful place to be. However, a polar bear would die on a tropical island. The bear would not be able to find its favorite food—seals—in the warm waters. The bear’s layer of fat and thick, white fur would make the bear too hot. A tropical island is the natural habitat for palm trees and parrots, but not polar bears.


ImageEvery time you take a breath, you are inhaling Earth’s atmosphere. You cannot see, smell, or taste Earth’s atmosphere. It is the air all around you. Other planets also have an atmosphere. An atmosphere is a blanket of gases that wraps around a planet or any other object in space.


Earth’s atmosphere is made up of a mix of gases called air. Air contains more nitrogen than any other gas. Nitrogen makes up 78 percent of the air. Oxygen, the gas that is most important for keeping you alive, makes up 21 percent.