thanks.. i really know nothing apart from that i enjoy doing it with hearing aid batteries myself.. there is something really satisfying watching the no-bounce battery. as for the reason, i always presumed it was because they take in oxygen. i can't find anything on the weight of discharged vs charged zinc air batteries... but they have always felt a teeny tiny bit lighter when used (i have no evidence!) and i am not sure about your water supposition... i know they -can- swell with water if it is too humid for example, but generally, don't they expel water as they discharge...? "Zn–air battery uses oxygen from the air, and hence, air holes in it are kept open for cell operation. Therefore, loss of water by evaporation through the holes is inevitable. When the water is depleted, the battery ceases to operate. There are two water consumption routes in Zn-air batteries, namely, active path (electrolysis) and passive path (evaporation and corrosion)." source: https://www.jecst.org/journal/view.php?number=300
regardless.. here is a nice video of the effect in hearing aid batteries.. https://www.facebook.com/BMHearing/videos/2037506353043179/ sorry the only link i can find worth sharing is on FB.