Sound is caused when objects vibrate (move back and forth very quickly). Vibrations create sound waves that can travel in all different directions through air, water, and lots of other materials. When sound waves are spread out, the sound we can hear is quiet. When they are clumped together, the sound is much louder. When sound waves go into your ear, they hit your eardrum and make it vibrate. The tiny vibrations move through your ear like a light shining through a long tunnel until they get to some nerves at the end of your ear. The nerves take them to your brain where they turn into the sound that you hear!
You can use a slinky to demonstrate how sound waves move. Have your child sit on the floor and hold onto one end of a slinky (a metal one will work best), stretch out the other end and sit down across from him/her. Then, while your child holds his/her end of the slinky still, move your end slightly from side to side. What happens? The motion creates 'waves'' that move down the slinky towards your child. Hold your end of the slinky still and let your child move his/her end and watch what happens to the waves. What happens to the waves if you both move your end at the same time? Sound waves travel in only one direction at a time, just like they do on the slinky.
This activity requires chewing and so could be either done during a meal or else as an experiment. To teach your kids more about sound, have them chew a bite of food, listen to the sound it makes, and compare the sounds made by soft foods and hard or crunchy foods. Try bananas, bread, crackers, chips, carrots, etc. Have them listen while you chew on the same foods. Were the sounds louder when they were chewing or when you were chewing? Ask them to predict whether the sounds will be louder or quieter if they cover their ears with their hands while chewing. Then let them experiment to see if their predictions were correct. The sounds we hear while chewing are actually louder when our ears are covered. Do you know why? There is a tube that connects each ear to your nose and throat called a Eustachian Tube. When you plug your ears from the outside, you can still hear sounds from inside of your mouth through this tube. Everything you hear sounds louder because the noises from outside your ears are being blocked out.
Thinking Scientifically: Your ears are home to the three smallest bones in your body. It is because of these three tiny bones that you can hear. They are called the hammer (its scientific name is malleus), anvil (or incus), and stirrup (or stapes).
Go here to see a labeled diagram of the ear: http://www.enchantedlearning.com/subjects/anatomy/ear/
For younger children: It may be challenging for little kids to stay quiet and remember what they have heard, so discuss what they hear as you go. See if they can imitate sounds they heard.
For older children: Have kids close their eyes while listening. Often when we hear something we look to see what it is. Closing our eyes forces us to figure out what the sound was using only our ears!
Select several containers of food from the kitchen such as a box of cereal, a can of soup, and a jar of rice. Shake each one and have your kids guess what it is without looking. Different foods make different sounds depending on what type of container they are in. For example, rice is hard and pointed and makes loud clinking sounds when the grains hit the sides of a container. Try putting a little bit of rice in a jar, some in a metal container, and some in a plastic container. Do they all make different sounds? Do you know why?
Thinking Scientifically: Different foods (and other materials) make different sounds when they knock against different materials (like glass, metal, or plastic). The different foods you had in each different kind of container made different sounds because some materials absorbed sound waves while others reflected them. For example, plastic absorbs much more sound than metal does, so the metal can of rice was louder than the plastic container. This is why rain sounds much louder on a metal roof than on a normal one.
Make other familiar noises by clapping your hands, tapping a pencil on a table, closing a book, crumple and/or tearing a piece of paper, turning on the faucet or computer monitor and have your kids guess what you are doing.
For younger children: Let your children shake the containers and listen to how each one sounds. After they have done this for a few minutes, have them wear a blindfold and guess which container you are shaking.