First we did the time measurements. Among the children, there were 4 watches, Ms. Irving had one, and I had brought one, plus an alarm clock with a second hand. Some watches had old-fashioned faces that could be observed by more than one person. I did the tapping, and the kids counted taps in a convenient 15-second interval. We got the following counts: 16, 16, 15, 16, 15, 16, 17, 17. The average is 16.0 counts in 15 seconds, or 0.94 second between beats.

Next we measured the distance to the wall.I had brought one 12-foot tape measure, and there were a number of them from the classroom. I explained how they could measure a few steps, and then count steps to the wall. The class spontaneously broke up into teams of 2 and 3, and went to work. These are the measurements we recorded:

[There were actually more counts, but not all got calibrated, and not all got written down - apologies to the kids that I missed]
Columns 2 and 3 are what was measured. From that, the distance to the wall was calculated (column 4).
In the time between the BANG and the returning echo, the sound has traveled to the wall and back. Since the echo is halfway between BANGS, the sound travels 4 times the distance to the wall in the time between BANGS on the can. Therefore the speed of sound is 4x what is in column 4, divided by the time we measured, 0.94 seconds. The result is written in column 5, which is the speed of sound in feet/second. The last column is this speed converted to miles per hour.

You can see from the table that there is a great spread in the estimates of the distance, which translates into a proportional spread in the final results.
Nevertheless, all results are in the right ballpark (about 1115 ft/s or 760 mph), and I think they all got the hang of how to do rough measurements quickly. Recall that we did the whole thing in a 1-hour time slot.

• Finding a good echo. The front of our school is not a single flat surface, but has walls at different distances (picture). When I tried out the measurement by myself, I started in the middle, but there were too many different echos. In the end, I had the best results by moving sideways, and found the best echo near the wall's end.
• Do a trial under real conditions. I did my trial on a Sunday morning. It was quiet, no cars. When I did it with the class, there was traffic noise, and there was a row of cars parked halfway to the wall. This made it noticably more difficult to hear the echo, but it still worked.
• I did have a small portable blackboard to collect results on. This was nice, but not absolutely necessary. Clipboards and paper would work also.
• There was a chainlink fence between where we stood and the school wall, and the gate in the fence was not right in front of where where we were working. Some teams did not perceive this as a problem: they angled for the gate, counting steps carefully, and angled back once they were through the gate. Keep an eye out for mistakes like that.