For background please read an older post Cartesian divers.
The Cartesian divers caught the fancy of the 5th graders and they were excited that it was a science experiment that they could explain it (we had briefly gone to a science exhibition a couple of weeks before) and on my suggestion three of them came forward to present it to the 3rd & 4th graders.
My guideline to them was to see if they can get the 3rd & 4th graders to figure it out themselves (by possibly conducting various experiments) rather than tell them the 'answer'.
As they planned the various experiments they were going to show (vial upside down, bottle open, closed and partially full, closed and full, etc) we found gaps in their own understanding and doing the experiments help fill these gaps. Then off we ran to the 3rd & 4th graders.
At a point in the presentation was a good number of the children thought it was the air pressure in the top of the bottle that pushes the vial down. The presenters promptly filled the bottle to the brim (so no air was in the top). Kam, one of the presenters, started a poll, how many thought the diver would go down and how many didn't. Then something I did not expect happened, he started asking why they thought so. This led to a discussion, however, not yielding the 'answer' the presenters had in their mind.
At this point one of the kids was just dying to tell what he knew, remembered the guideline (teacher says guideline, child hears instruction), and started saying - 'look more closely, can't you see'. The other just closed his mouth so he would not speak!