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.

Most illustrations have captions. A caption identifies or explains the illustration. Some captions are short; others are longer and more descriptive. Diagrams often have labels that identify parts of the organism or the order of steps in a process, such as the labels in Figure 19.

Learning with Illustrations  An illustration of an organism shows that organism from a particular side.

In order to understand the illustration, you may need to identify the front (anterior) end, the tail (posterior) end, the underside (ventral), and the back (dorsal) side, as shown in Figure 20.

To make a model, you first have to get a basic idea about the structure or process involved. For example, 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. The different kinds and sizes of pasta shown in Figure 21 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.

Measuring in SI
The International System (SI) of Measurement is accepted as the standard for measurement throughout most of the world. Four of the base units in SI are the meter, liter, kilogram, and second.

The size of the unit can be determined from the prefix used with the base unit name. Look at Figure 22 for some common metric prefixes and their meanings. 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. The metric system is convenient because its 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 units, 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).

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. A millimeter is one-thousandth of a meter (0.001).

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.

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. 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 triple beam balance similar to the one shown in 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 along 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.

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 number of organisms of a population for research. Information discovered with the small sample may then be applied to the whole population. For example, scientists may take a small number of mice from a field to study the effects of day length on reproductive rate. This information could be applied to the population as a whole.

Estimating  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. Here is a familiar example. Have you ever tried to guess how many kernels of popcorn were in a sealed jar? If you did, you were estimating. What if you knew the jar of popcorn held one liter (1000 mL)? If you knew that 60 popcorn kernels would fit in a 100-milliliter jar, how many kernels would you estimate to be in the one-liter jar? If you said about 600 kernels, your estimate would be close to the actual number of popcorn kernels.

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 black-eyed Susans, the field could be marked off in a large grid of 1-meter squares. To determine the total population of the field, the number of organisms in one square-meter sample can be multiplied by the total number of square centimeters in the field.