Echoes are a good starting point for learning about the speed of sound, however I made the mistake of trying (too soon) to teach my three-year-old son about echoes.

We were in the front yard of his grandparents house in South Mississippi. Several cedar trees bordered the yard. I told him to listen to the sound that bounced off the tree trunks (the echoes) after I clapped my hands. The trees did their part. The echoes were clear and pronounced.

My son was terrified!

He lamented, "The tacco makes the echo!"

(Rhyming may be a linguistic form of the echo phenomenon.)

Then he realized, that his new word was already taken, i.e., the Spanish word taco.

Next came, and this time it stuck.

"The hocco makes the echo!"

He didn't buy into my physics. (He's been kind of down on the subject ever since!) As far as he was concerned there were some hard-to-see creatures (the hoccos) in the woods that mimicked any sound you made. That he could not see them was very upsetting. They must be very small, indeed.

For a first person (victim turned writer) fictionalized version of this adventure see The Hocco Makes the Echo. (Get some chips and refill your drink before clicking. There's a surprise ending.)

Notwithstanding, echoes are still a good starting point for learning about the speed of sound. Once you realize it takes a noticeable amount of time for sound to travel from one point to another (and back) then you are one step removed from thinking up experiments to learn new things about your environment.

For the sake of this discussion I'll go one step further and stipulate that you and your fellow islanders are not in touch with the outside world, and its ideas, so what you know about physics is based on local experiments.

For fun, lets postulate that, after a lot of study and deliberation, it has been concluded that light and sound propagate at the same speed, a speed which is yet is to be determined.

Then one day several of you observe a shiny silent bird-like-object moving gracefully high-up across the sky. It has a long extended white tail. (On more careful examination the tail appears to consist of at least two more-or-less parallel parts.) After a while you perceive a faint roaring sound (with no visible source) that seems to be originating from an area somewhat behind the silent bird like object. Nobody can see anything where the sound seems to be coming from, so the initial consensus is that the source must be very compact or invisible. After a while you begin to call the Silent Bird Like Object a SBLO and the Tiny (Or Invisible) Roarer a TOIR, pronounced "tore." (In this adventure everybody speaks English and knows how to make acronyms.)

Time passes. More SBLOs cross your sky, and always followed by the TOIRs.

Sometimes the SBLOs seem closer to you (closer to the ground) and you note that the SBLO and TOIR seem closer to one another and that the TOIRs in these cases are louder, but still, nobody has ever seen a TOIR.

This could go on and on but the real purpose in starting it at all was to suggest that our ideas about how fast information comes to us (light, sound, etc.) affects our interpretation of our observations. In this adventure, islanders who steadfastly hold to the opinion that sound and light travel the same speed will tend to wrestle with two separate issues. How can visible, obviously real, objects (the SBLOs) move through the air without making any sound and what is the real nature of the mysterious TOIRs. (Either those enigmatic objects are very energetic to produce so much sound from such a small volume or they posess some form of visiblity cloaking (light bending perhaps) that is yet to be understood.

If research groups are set up to work on various aspects of these mysteries, they won't have to worry about running out of work. On the other hand, once it has come to be realized that sound travels much slower than light (as perhaps determined from newly designed lip-sync delay studies) they will be able to conclude that the trailing roaring sound is actually a speed-of-light versus speed-of-sound accoustical aberration. At that point the SBLO TOIR schizophrenia can be excised.

For an article written to suggest how what we think we know about the speed of light affects our interpretation of certain astronomical oddities, see: A Ritzian Interpretation of Variable Stars

Warning! Everything in the recommended article may be wrong. Even so, I now submit, after having just read the whole hocco story, that the null-result of the Michelson-Morely experiment might have been very scary and that great feats of imagination may have been applied to make everybody feel better.