For a simple third-grade activity, teach the students how something as trivial as added sugar can weigh down their favorite soft drink. Use a can of regular soft drink and a can of the diet version; set both cans in an aquarium filled with water but no fish. The children may notice that the regular version of the soft drink sinks, while the diet soda can floats on the surface; the added weight of the sugar in the regular cola makes it heavier than the water around it. Since most diet soft drinks use artificial sweeteners that are more concentrated than sugar, they weigh less. As more matter is packed into the can, the volume of the can increases the mass and affects the density of the can. Explain how an object will float if it has less density than the water it's in and will sink if it has a greater amount.
Have your third graders use a scale to compare volume, weight and mass and to explain the different types of measurements. Use a set of small weights that measures from 1 ounce to 1 pound; place the weights on a scale to teach students how to recognize different masses and how the scale is influenced when more or less weight is added. They can take turns using items such as coins or books to discover which weighs more and to estimate how much something weighs. In this manner, the third graders will also learn which objects have a greater mass.
If you have a helium tank in your classroom, invite the students to blow up and tie balloons and, one-by-one, drop the balloons to the ground. Your students will notice that the balloons fall to the ground at a speed that depends on the amount of air in the balloon. You can explain the relationship of the amount of air in the balloon to the density of the rubber surrounding it, which makes it heavier.
Fill several balloons with helium and watch them float, defying gravity. As children marvel over the magic, you can explain how helium, pound for pound, has less density than air, and so it actually moves upwards, bringing air down. You can demonstrate the same concept by filling one balloon moderately with helium and another more full and letting go of both balloons. The balloon with just enough helium will float up slowly, while the other will move faster; this takes place because the slower balloon has less density of helium to lift it past the air.
Third-grade children also benefit from learning about the metric system of measurement for weights. A good way to teach this lesson is to use a digital scale that can measure grams and place a 1-pound weight on the scale. The result is 453.6 grams, rounded up. An ounce weight equals 28.3, also rounded. Have the children take turns using different objects to understand the differences and the larger figures involved in metric measurements.