Find several different pairs of containers that are of different shapes but hold about the same amount of liquid. Have the students then guess which containers hold about the same amount, and then have the students fill up one of the containers with water and pour it into the corresponding container that they believed to hold the same amount. This will help the students to understand that even though the containers are different shapes, they have the same capacity.
To go over comparing different capacities, begin with four clear plastic cups of the same size. You will also need a clear measuring cup that measures up to one cup. Fill the cups up with 1/3,1/2, 3/4 and one cup of water, rice or something similar. Have the students compare the cups to discover if they have the same amount, less or more. Have the students look at all of the cups side by side to compare the size of the cups. Pour the contents of a cup into another and show that they can hold the same amount.
Fill a clear container with gummy bears, jelly beans or some other fun candy. Have each of the students guess how many of the candies are in the container. The students can do this either by their name on a chalkboard or on a piece of paper. Then have the children help you count the candies as you take them out of the container. Go over with the students how these results will be different if the candies were bigger or smaller. Once the activity is over, allow the students to eat the candies.
Fill a tall, glass, cylindrical vase only partially with water. Have the students use a ruler to measure how deep the water is. Discuss how full the container is and if it is deep or shallow. Then have them place clean rocks into the container and discuss what happens to the water level: is it higher, deeper or more shallow?