When you make observations in science, you may find it helpful first to examine the entire object or situation. Then, using your senses of sight, touch, and hearing, examine the object in detail. Write down everything you observe.

Scientists often use their observations to make inferences. An inference is an attempt to explain or interpret observations or to determine what caused what you observed. For example, if you observed a CLOSED sign in a store window around noon, you might infer that the owner is taking a lunch break. But, perhaps the owner has a doctor's appointment or has a business meeting in another town. The only way to be sure your inference is correct is to investigate further.

When making an inference, be certain to make accurate observations and to record them carefully. Collect all the information you can. Then, based on everything you know, try to explain or interpret what you observed. If possible, investigate further to determine whether your inference is correct. What can you infer from observing the behavior of a cat that is crouched and ready to pounce?

Comparing and Contrasting
Observations can be analyzed and then organized by noting the similarities and differences between two or more objects or situations. When you examine objects or situations to determine similarities, you are comparing. Contrasting is looking at similar objects or situations for differences.

Suppose you were asked to compare and contrast a grasshopper and a dragonfly. You would start by making your observations. You then divide a piece of paper into two columns. List ways the insects are similar in one column and ways they are different in the other. After completing your lists, you report your findings in a table or in a graph.

Similarities you might point out are that both have three body parts, two pairs of wings, and chewing mouthparts. Differences might include large hind legs on the grasshopper, small legs on the dragonfly; wings held close to the body in the grasshopper, wings held outspread in the dragonfly.

Recognizing Cause and Effect
Have you ever observed something happen and then tried to figure out why or how it came about? If so, you have observed an event and inferred a reason for the event. The event or result of an action is an effect, and the reason for the event is the cause.

Suppose that every time your teacher fed fish in a classroom aquarium, she tapped the food container on the edge. Then, one day she tapped the edge of the aquarium to make a point about an ecology lesson. You observe the fish swim to the surface of the aquarium to feed.

What is the effect and what would you infer was the cause? The effect is the fish swimming to the surface of the aquarium. You might infer the cause to be the teacher tapping on the edge of the aquarium. In determining cause and effect, you have made a logical inference based on careful observations.

Perhaps the fish swam to the surface because they reacted to the teacher's waving hand or for some other reason. When scientists are unsure of the cause for a certain event, they often design controlled experiments to determine what caused their observations. Although you have made a sound judgment, you would have to perform an experiment to be certain that it was the tapping that caused the effect you observed.

Interpreting Scientific Illustrations
Illustrations are included in your textbook to help you understand, interpret, and remember what you read. Whenever you encounter an illustration, examine it carefully and read the caption. The caption explains or identifies the illustration.

Some illustrations are designed to show you how the internal parts of a structure are arranged. Look at the illustrations below of a squash. The squash has been cut lengthwise so that it shows a section that runs along the length of the squash. This type of illustration is called a longitudinal section. Cutting the squash crosswise at right angles to the length produces a cross section.

In your reading and examination of the illustrations, you will sometimes see terms that refer to the orientation of an organism. The word dorsal refers to the upper side or back of an animal. Ventral refers to the lower side or belly of the animal. The illustration of the shark shows that it has both dorsal and ventral sides.

Symmetry refers to a similarity or likeness of parts. Many organisms and objects have symmetry. When something can be divided into two similar parts lengthwise, it has bilateral symmetry. Look at the illustration of the butterfly on the next page. The right side of the butterfly looks very similar to the left side. It has bilateral symmetry.

Other organisms and objects have radial symmetry. Radial symmetry is the arrangement of similar parts around a central point. The anemone in the figure has radial symmetry. It 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. Study the sponge. Regardless of how you try to divide a sponge, you cannot divide it into two parts that look alike.

Calculating Magnification
Objects viewed under the microscope appear larger than normal because they are magnified. Total magnification describes how much larger an object appears when viewed through the microscope.

Look for a number marked with an X on the eyepiece, the low-power objective, and the high-power objective. The X stands for how many times the lens of each microscope part magnifies an object.

To calculate total magnification, multiply the number on the eyepiece by the number on the objective. For example, if the eyepiece magnification is 43, the low-power objective magnification is 10X, and the high-power objective magnification is 40X:

(a)	then total magnification under low power is 4X for the eyepiece times 10X for the low-power objective = 40 (4 X 10 = 40).
(b)	then total magnification under high power is 4 X 40 = 160.

To measure the field of view of a microscope, you must use a unit called a micrometer. A micrometer equals 0.001 mm; in other words, there are 1000 micrometers in a millimeter. Place a millimeter section of a plastic ruler over the central opening of your microscope stage. Using low power, locate the measured lines of the ruler in the center of the field of view. Move the ruler so that one of the lines representing a millimeter is visible at one edge of the field of view.

Remember that the distance between two lines is one millimeter, and estimate the diameter in millimeters of the field of view on low power. Calculate the diameter in micrometers. For example, if the distance is 1.5 mm, then the diameter of the field of view at low power is 1500 mm. [1.5 X 1000]

To calculate the diameter of the high-power field, divide the magnification of your high power (40X) by the magnification of the low power (10X); 40/10 = 4. Then, divide the diameter of the low-power field in micrometers (1500 µm) by this quotient (4). The answer is the diameter of the high-power field in micrometers. In this example, the diameter of the high-power field is 1500/4 = 375 mm.

You can calculate the diameters of microscopic specimens, such as pollen grains or amoebas, viewed under low and high power by estimating how many of them could fit end to end across the field of view. Divide the diameter of the field of view by the number of specimens.