A lever is a board or a bar that turns on a fixed support called a fulcrum. Fingernail clippers are an example of levers. The force exerted on the handle of the clippers compresses the blades of the clippers so that they trim the fingernail. You might want to look at a pair of clippers with your children, and see if they can identify the fulcrum (the pivot joint between the two parts, in this case).
There are three types of levers: first, second, and third class. Nail clippers are first class levers. You can make your own first class lever, using a ruler with a pencil to work as the fulcrum. Center the ruler over the pencil, and set a small object or weight (this is called the 'load') on one end of the ruler. When you push on the opposite end of the ruler (the force you exert is called the 'effort'), the weight is lifted. You might want to mention other types of first class levers to your children; a seesaw is one example.
A second class lever has the load located in the middle of the ruler, with the fulcrum on one side and the effort on the other. Using a spring scale that measures in newtons, you can identify the mechanical advantage. If you have not already done so, you will need to find the weight of your load. Hook the load onto the spring scale, and record the weight in newtons. Next, set the ruler ('lever arm'), fulcrum, and load into position so that you form a second class lever. The lever arm should have one end resting on the fulcrum, with the load placed at the center of the lever arm. Hook the spring scale to the lever arm at the end that is opposite the fulcrum, then pull up on the spring scale to lift the load. Record the effort force required to lift the load. To find the mechanical advantage of your lever, divide the weight of the load by the effort force that was required to lift it.
For further study of second class levers, you might want to also measure the length of the lever arm (ruler) from the fulcrum to the load, and from the load to the spring scale. Use the spring scale to act as the effort force again. After you have repeated this step several times, changing the position of the load and measuring it each time, analyze your results. Does the effort force required to lift the load change when the load is moved closer to the fulcrum? You should be able to observe that the effort force is reduced as the load is placed closer to the fulcrum. This is because the mechanical advantage increases as the distance between the load and fulcrum decreases.
Third class levers have the effort located between the fulcrum and the load. To experiment with effort force reduction, set up your lever with the fulcrum on one end and the load on the other. The spring scale will be used in the middle. Discuss with your children whether they think the effort force will be reduced when it is moved closer to the fulcrum, or further away. As with the second class lever, pull up on the spring scale and record the effort force required (you will need to put your finger on the end with the fulcrum, to act as a pivot point). Also measure the distance on the lever arm from the fulcrum to the load, and from the fulcrum to the spring scale. Repeat the process several times, moving the effort closer to and further from the fulcrum. When you are finished, analyze your results. How did the effort force change when it was moved closer to or further from the fulcrum? The effort force that is required to lift the load is reduced as the effort location is moved away from the fulcrum.