Have you ever stood along side a road while a car passed by with it's horn blowing? If so, you probably noticed that the pitch of the sound was higher when the car was approaching than after it passed. This is called the Doppler effect.
Go out in the country with someone who can drive a car. Take along a battery-operated tape recorder. If you have a pair of walkie-talkies, take them along too.
Find a long, straight road where there are no houses or other cars. Find a place where you can safely sit or stand off to the side of the road with the tape recorder.
Have the driver drive past you three times, once at 20, once at 30, and once at 40 miles per hour. Each time have the driver blow the horn for a few seconds as the car passes. Your job is to start and stop the tape recorder at the right times to record the sounds. You can also record on the tape in your own voice what's happening. Also make a recording of what the horn sounds like when the car is not moving. Have the driver drive past without telling you the speed. Can you estimate the speed from the pitch of the sound?
The police use radar to measure the speed of cars. Radar works by the Doppler effect except that it uses radio waves instead of sound waves. Where do the radio waves come from?
The Doppler effect occurs for all kinds of waves. It happens when either the source of the waves or the receiver of the waves is moving. Could you observe the Doppler effect of water waves if you were in a moving boat?