Use the knife and cut one of the cacti in two pieces. Let each student feel the inside of the cactus. Point out that the plant is wet inside because that is where it stores it's water. Let them see the big root that runs down the center. People can cut into these plants to drink the water inside.
People can eat the inside of cacti plants for food to stay alive.
They also use the big Sagauro cacti as shields from the sun. Ask children if they know of any other plants that store water. Place a small cactus and a small potted flower in warm place in the room. Tell the students to leave them both in that spot without water and in a week you can check them to see how well storing water works. Give each group of four children a cactus plant and a magnifying glass.