Kindergartners are not capable of grasping abstract concepts, so they tend to think of big and heavy, and small and light as synonyms. For them to understand the concrete fact that small objects can be heavy you must find a way to get them physically involved with the weight of objects so that they can see the relationship between weight and size. This will also teach them that the weight of an object is not always as it appears.
Use balancing scales to explain heavy and light. Start by putting one marble on one side of the scale and two or three on the other so that students can see how the scale responds to weight, the way it sinks lower on the heavy sides and goes up on the lighter side. Then allow students to experiment with putting small objects on each side of the scales to see which is heavier and which is lighter. Give them objects like balloons, hot wheels cars, golf balls, small dolls, food products and other small objects.
Teach children the signs for heavy and light in American Sign Language. Then show students heavy and light objects and have them hold up their hands with the sign for heavy or light, depending on the object you hold up. For example, first hold up a bowling ball, and have students give the sign for heavy. When all students have responded correctly, hold up a baseball and have students give the sign for light. Repeat with other objects like heavy and light fruits, perhaps a watermelon and an apple and heavy and light toys.
Give students two laundry baskets and a table full of objects. Make sure that you have a good variety of objects. You should have small things that are heavy, like a paper weight, and large things that are light, like a beach ball. It is important to make sure that children do not confuse big and small with heavy and light. Have students sort objects into the two laundry baskets, placing all the light objects in one and all the heavy objects in the other. At the end, have students justify their classification for objects that you would have sorted differently than they did. Correct their logic if it is wrong.
Make or print a heavy and light colouring book. A Concept book is available at the K-3 Teacher's Resources website that you can download and print for free. Have students colour pages that already have light and heavy comparison and then fill in the blank pages with comparisons of their choice. Let students decorate the pages with crayons, markers and any other craft supplies you choose to provide.
Show and Tell
Set up two show and tell days for children during the week or weeks that you are teaching the concepts of heavy and light to students. Have students bring a heavy and a light object to show the class. Students can tell about the thing that they bring, where they got it, why it is important to them and why they brought it as an example of heavy or light. Make sure that if they bring a heavy item on the first day, they bring a light item on the second day so that you can check for comprehension of both concepts.