Get a CD collection of sound effects or download some from a website. You can now play a game in which you play the sounds through a speaker. Ask students to write on a piece of paper what they think each sound is. The student with the highest number of correct answers wins. After this exercise, discuss students' answers as a way to introduce the nature of sound and hearing.
Explain to the students that sound waves travel through the air in the same way that waves move through water. This means that you can make a model of how sound waves work with water. To do this, you need a digital camera, a computer, a large pan of water to which you have added a bit of white paint, a marble and geometric blocks of various shapes. Place the camera so it is recording the surface of the water. Have the children place blocks around the center and then drop a marble in. Play the video of these drops back at slow speed on the computer so they can see the echo of the "sound waves" bouncing around from the objects placed in the water.
Third-graders will be fascinated by the concept of hearing cones. Provide them with construction paper, safety scissors and tape so they can create large cones they can hold to their ears. Take them outside so they can test their effectiveness. Explain how the cones gather and focus the sounds in the same way that a magnifying glass focuses light on a point.
Another activity for young students is to create cup phones. Just use something pointy to punch a small hole in the ends of two cups. Run a string through these holes and secure them using paper clips. When the kids hold the cup so the string is tight, they can speak through one end and hear through the other. Ask them how they think this works.