In order to generate questions, students need previous knowledge of the subject. For example, most fourth-graders will know that it is more difficult to stop a car on a wet or icy street than on a dry one. Ask students whether toy cars will slide down inclined planes faster if the plane has been waxed or covered with sandpaper. Instruct them to perform an experiment by timing how long it takes cars to slide down the planes with different surfaces. Ask them to record their results in a notebook. Most students will understand that rougher surfaces will cause moving objects to slow down. Explain that rougher surfaces have more friction. Fourth-grade students are also expected to develop an understanding of gravity. It is important to tell them that in a perfect vacuum, a feather and a brick will fall at the same rate and that wind resistance, which physicists call drag, is caused by molecules of air striking an object as it moves. Appropriate questions are whether heavy objects fall faster than lighter ones, and whether the surface area of an object affects the speed with which it falls. Students can answer these questions by dropping objects of varying weights or dropping objects with the same weights but with different surface areas. Ask them to explain their results in terms of wind resistance.
Fourth-graders know that plants need water, light and good soil. Ask how much water a certain kind of plant needs. Lead a discussion about the best way to answer this question. Explain that a good experiment varies one factor at a time. If the purpose of this experiment is to investigate the effects of water, then all the other factors, such as sun exposure and soil quality, must be the same. This kind of project provides a good opportunity to use the tools of science. Meter sticks and scales can be used to measure and record data. Students can also be encouraged to keep lab notebooks -- another important scientific tool -- to record their observations.
By fourth grade, most students will know that their parents put antifreeze in the car to keep the water in the radiator from either freezing or boiling over. Guide the students to ask if adding other substances to water, such as salt -- whose chemical formula is NaCl -- will lower the freezing point of water. Ask if adding even more salt will further lower the freezing point. Ask if other substances, such as potassium chloride, will also lower the freezing point. Try to generalize from the experiments about why adding compounds lowers the freezing point.
If students are allowed to keep small animals such as mice in the classroom, they can perform a nutrition experiment. Suppose students want to know which kinds of fat are the healthiest. One group of animals can be fed hydrogenated fat, another group can be fed saturated fat from lard and a third group can be fed olive oil. This experiment provides another opportunity for discussing fair testing: great care must be taken to ensure that all three groups get the same amount of calories and nutrients. During the course of the school year, animals can be weighed and their activity on running wheels can be measured. Students can also look for signs of aging, such as difficulty moving, and record which animals showed signs of aging first.