The girls have been enjoying goofing around the bathtub with a 32oz yogurt container and a wet washcloth. My oldest discovered that if you invert the full container with the wet washcloth held tightly over the opening, the water doesn't flow out. They were having so much fun with it, and (even if I did understand what was going on) I didn't want to spoil it by explaining.
But *I* want to know.... what just happened?
Give me a few minutes while I caffeinate.
They don't even need the washcloth over the opening. The water does not flow out, because there is nothing to take up that space that it is occupying in the container. If air cannot get in, then the water can not come out. As long as the rim of the container is under water, and there are no holes in the container, the water will have to stay in.
Yes, that's it. Water can get through the washcloth pretty well, but air can't. I just played around with this a little myself and found that if you pull the washcloth away from the container just a tiny bit in one spot along the edge, so air can get in, water will come pouring out through the washcloth.
I was thinking about this more last night and puzzling over how to explain why the water can't come out. Why can't it leave an empty space behind? And how does it "know" there would be an empty space? What actually prevents it from moving? The answers to those questions weren't really clear to me at first, but after thinking it over and talking about it with DP, I feel like I understand now.
Here's the way of thinking about it that makes sense to me:
For the water to come out, there has to be space for it to flow into. But the space outside the container is already full. It's full of air. If air could get into the container, then some of it would shift into there as the water came out, making room for the water outside the container. But if there's nowhere for the air to go, the water can't just push it aside into another space; it has to compress it. In other words, the water pressure has to be higher than the air pressure. But it's not; the air is pushing against the water harder than the water is pushing against the air. DP tells me that a column of water needs to be 30 feet high for the pressure in the column to be more than the air pressure. (It doesn't matter how wide the column is, because if it's wider there's not only more water pressing down but also more air pressing up, so it always stays even.) So if your kids had a 30 foot tall yogurt container and they turned it upside down with a washcloth over it, water would come out through the washcloth, leaving a vacuum behind.