An experiment was conducted that involved driving a car from A to B at a constant speed over a recorder, with the horn sounding. It was observed that the recorded sound from the horn varied in pitch as the car approached and then moved away from the recorder.
Explain why there is a difference in the observed sound.
Zaloum's Physics Discussion 
As the car is travelling towards the sound recorder the wave fronts will be closer together, because each successive wave crest will be emitted closer to the observer from the previous wave. Thus making the waves closer together as you are traveling towards the tape recorder also decreasing the time of arrival(getting to the recorder quicker). As a result of this the tape recorder will be recording a high frequency.
As the car is traveling away from the tape recorder the wave fronts will be further apart when traveling to the recorder, because the source of sound is traveling away from it. Each successive wave crest will be emitted further away from the previous on. This increases the time of arrival (takes longer to get to the recorder) Resulting in a decreased frequency recorded by the tape recorder.
Romana,
This is a very nicely explained answer, well done. I can’t think of how it could be improved.
Everyone else, there are other ways of answering this question so you can feel welcome to have a go at answering in a way that best spins your wheels.
Tony