Interpreting Scientific Illustrations
As you read a science textbook, you will see many drawings, diagrams, and photographs. Illustrations help you to understand what you read. Some illustrations are included to help you understand an idea that you can't see easily by yourself. For instance, we can't see atoms, but we can look at a diagram of an atom and that helps us to understand some things about atoms. Seeing something often helps you remember more easily. Illustrations also provide examples that clarify difficult concepts or give additional information about the topic you are studying. Maps, for example, help you to locate places that may be described in the text.

Examples
Captions and Labels  Most illustrations have captions. A caption is a comment that identifies or explains the illustration. Diagrams, such as Figure 20, often have labels that identify parts of the organism or the order of steps in a process.

Learning with Illustrations  An illustration of an organism shows that organism from a particular view or orientation. In order to understand the illustration, you may need to identify the front (anterior) end, tail (posterior) end, the underside (ventral), and the back (dorsal) side as shown in Figure 20.

You might also check for symmetry. The shark in Figure 21 has bilateral symmetry. This means that drawing an imaginary line through the center of the animal from the anterior to posterior end forms two mirror images.

An object or organism such as a hydra can be divided anywhere through the center into similar parts.

Some organisms and objects cannot be divided into two similar parts. If an organism or object cannot be divided, it is asymmetrical. Regardless of how you try to divide a natural sponge, you cannot divide it into two parts that look alike.

Some illustrations enable you to see the inside of an organism or object. These illustrations are called sections. Figure 22 also illustrates some common sections.

Look at all illustrations carefully. Read captions and labels so that you understand exactly what the illustration is showing you.

Making Models
Have you ever worked on a model car or plane or rocket? These models look, and sometimes work, much like the real thing, but they are often on a different scale than the real thing. In science, models are used to help simplify large or small processes or structures that otherwise would be difficult to see and understand. Your understanding of a structure or process is enhanced when you work with materials to make a model that shows the basic features of the structure or process.

Example  In order to make a model, you first have to get a basic idea about the structure or process involved. You decide to make a model to show the differences in size of arteries, veins, and capillaries. First, read about these structures. All three are hollow tubes. Arteries are round and thick. Veins are flat and have thinner walls than arteries. Capillaries are small.

Now, decide what you can use for your model. Common materials are often best and cheapest to work with when making models. Different kinds and sizes of pasta might work for these models. Different sizes of rubber tubing might do just as well. Cut and glue the different noodles or tubing onto thick paper so the openings can be seen. Then label each. Now you have a simple, easy-to-understand model showing the differences in size of arteries, veins, and capillaries.

What other scientific ideas might a model help you to understand? A model of a molecule can be made from gumdrops (using different colors for the different elements present) and toothpicks (to show different chemical bonds). A working model of a volcano can be made from clay, a small amount of baking soda, vinegar, and a bottle cap. Other models can be devised on a computer. Some models are mathematical and are represented by equations.

Measuring in SI
The metric system is a system of measurement developed by a group of scientists in 1795. It helps scientists avoid problems by providing standard measurements that all scientists around the world can understand. A modern form of the metric system, called the International System, or SI, was adopted for worldwide use in 1960.

The metric system is convenient because unit sizes vary by multiples of 10. When changing from smaller units to larger units, divide by 10. When changing from larger units to smaller, multiply by 10. For example, to convert millimeters to centimeters, divide the millimeters by 10. To convert 30 millimeters to centimeters, divide 30 by 10 (30 millimeters equal 3 centimeters).

Prefixes are used to name units. Look at Figure 23 for some common metric prefixes and their meanings. Do you see how the prefix kilo- attached to the unit gram is kilogram, or 1000 grams? The prefix deci- attached to the unit meter is decimeter, or one-tenth (0.1) of a meter.

Examples
Length  You have probably measured lengths or distances many times. The meter is the SI unit used to measure length. A baseball bat is about one meter long. When measuring smaller lengths, the meter is divided into smaller units called centimeters and millimeters. A centimeter is one-hundredth (0.01) of a meter, which is about the size of the width of the fingernail on your ring finger. A millimeter is one-thousandth of a meter (0.001), about the thickness of a dime.

Most metric rulers have lines indicating centimeters and millimeters. The centimeter lines are the longer, numbered lines; the shorter lines are millimeter lines. When using a metric ruler, line up the 0-centimeter mark with the end of the object being measured, and read the number of the unit where the object ends, in this instance 4.5 cm.

Surface Area
Units of length are also used to measure surface area. The standard unit of area is the square meter (m²). A square that's one meter long on each side has a surface area of one square meter. Similarly, a square centimeter, (cm²), shown in Figure 24, is one centimeter long on each side. The surface area of an object is determined by multiplying the length times the width.

Volume
The volume of a rectangular solid is also calculated using units of length. The cubic meter (m³) is the standard SI unit of volume. A cubic meter is a cube one meter on each side. You can determine the volume of rectangular solids by multiplying length times width times height.

Liquid Volume
During science activities, you will measure liquids using beakers and graduated cylinders marked in milliliters, as illustrated in Figure 25. A graduated cylinder is a cylindrical container marked with lines from bottom to top.

Liquid volume is measured using a unit called a liter. A liter has the volume of 1000 cubic centimeters. Because the prefix milli- means thousandth (0.001), a milliliter equals one cubic centimeter. One milliliter of liquid would completely fill a cube measuring one centimeter on each side.

Mass
Scientists use balances to find the mass of objects in grams. You will use a beam balance similar to Figure 26. Notice that on one side of the balance is a pan and on the other side is a set of beams. Each beam has an object of a known mass called a rider that slides on the beam.

Before you find the mass of an object, set the balance to zero by sliding all the riders back to the zero point. Check the pointer on the right to make sure it swings an equal distance above and below the zero point on the scale. If the swing is unequal, find and turn the adjusting screw until you have an equal swing.

Place an object on the pan. Slide the rider with the largest mass along its beam until the pointer drops below zero. Then move it back one notch. Repeat the process on each beam until the pointer swings an equal distance above and below the zero point. Add the masses on each beam to find the mass of the object.

You should never place a hot object or pour chemicals directly onto the pan. Instead, find the mass of a clean beaker or a glass jar. Place the dry or liquid chemicals in the container. Then find the combined mass of the container and the chemicals. Calculate the mass of the chemicals by subtracting the mass of the empty container from the combined mass.

Predicting When you apply a hypothesis, or general explanation, to a specific situation, you predict something about that situation. First, you must identify which hypothesis fits the situation you are considering.

Examples  People use prediction to make everyday decisions. Based on previous observations and experiences, you may form a hypothesis that if it is wintertime, then temperatures will be lower. From past experience in your area, temperatures are lowest in February. You may then use this hypothesis to predict specific temperatures and weather for the month of February in advance. Someone could use these predictions to plan to set aside more money for heating bills during that month.

Using Numbers
When working with large populations of organisms, scientists usually cannot observe or study every organism in the population. Instead, they use a sample or a portion of the population. To sample is to take a small representative portion of organisms of a population for research. By making careful observations or manipulating variables within a portion of a group, information is discovered and conclusions are drawn that might then be applied to the whole population.

Scientific work also involves estimating. To estimate is to make a judgment about the size of something or the number of something without actually measuring or counting every member of a population.

Examples
Suppose you are trying to determine the effect of a specific nutrient on the growth of black-eyed Susans. It would be impossible to test the entire population of black-eyed Susans, so you would select part of the population for your experiment. Through careful experimentation and observation on a sample of the population, you could generalize the effect of the chemical on the entire population.

Here is a more familiar example. Have you ever tried to guess how many beans were in a sealed jar? If you did, you were estimating. What if you knew the jar of beans held one liter (1000 mL)? If you knew that 30 beans would fit in a 100-milliliter jar, how many beans would you estimate to be in the one-liter jar? If you said about 300 beans, your estimate would be close to the actual number of beans. Can you estimate how many jelly beans are on the cookie sheet in Figure 27?

Scientists use a similar process to estimate populations of organisms from bacteria to buffalo. Scientists count the actual number of organisms in a small sample and then estimate the number of organisms in a larger area. For example, if a scientist wanted to count the number of bacterial colonies in a petri dish, a microscope could be used to count the number of organisms in a one-square-centimeter sample. To determine the total population of the culture, the number of organisms in the square-centimeter sample is multiplied by the total number of square centimeters in the culture.