Who Does It?

So, who changes the water cooler jug? We had an unending supply of water jugs in an office I once worked. I could bring my cup to the cooler at any time during the workday, pull down the lever and always heard the inevitable glug, glug. And it made me wonder, “Agua supply is getting low. What do I do if I take the last of this water? Do I, my 5 foot, 3 inch person, have to pick up one of those huge jugs placed at the side of the water cooler and flip it onto the cooler? How could I do that?” You just know (or at least I do) there would be massive spillage. So, I finished filling my cup and sneaked away to my office. I waited two hours. Then, I coyly went back and peeked at the water level of the cooler. Almost to the top! Cool! Someone else took on the burden. But who? Who are the mystery people who lug these massive blue bottles upside down onto the water cooler? I sat right by this watering hole and I never heard anyone grunting in effort and then hearing a click of the juggernaut going into the water contraption. And, what happens to the small amount of water that was in the prior bottle? Is it thrown out? Is it put on the plants? Does the changer just chug it?