immune system

A gas with no color, taste, or smell that is necessary for all human and animal life. It makes up about 21 percent of the Earth's atmosphere and is essential for the process of breathing.

Hydrogen is a chemical element that is the lightest and most common gas in the universe. It has no color or smell and combines with oxygen to make water (H2O).

Nitrogen is a colorless and odorless gas that makes up most of the Earth's atmosphere. It is a chemical element that is essential for all living things to grow, especially plants.

A polymer is a large molecule made of many smaller units that are connected in a long chain. These materials can be natural, like rubber or wood, or synthetic, like plastic and nylon.

A prime number is a whole number greater than 1 that cannot be made by multiplying other whole numbers. It can only be divided evenly by itself and the number 1.

A ratio is a way to compare two amounts of things. It shows how much of one thing there is compared to another thing, like 2 parts water to 1 part juice.

The perimeter is the total distance around the outside edge of a shape or an area. In math and science, it is found by adding the lengths of all the sides of an object.

The diameter is the length of a straight line that goes through the center of a circle or a round object, connecting two points on its edge. It is the measurement that tells you how wide a circle or sphere is at its widest point.

A radius is a straight line from the center of a circle to its outer edge. It is also used to describe the distance from a center point in any direction to mark an area.

The circumference is the distance around the outside edge of a circle or a round object. It is a measurement that tells you how long the boundary of a round shape is.