The above idea generated a lot of comments.
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A good seal needs to be maintained around the straws. When making the fountain use playdough, clay, or anything that can provide a seal. We put the playdough around both sides of the lid and around the straw. It is hard to maintain the seals. Just give everyone plenty to work with and they'll get the fountain to work.
We got this project to work perfectly every time by using modeling clay, and only on the outside of the lid.
We drilled 1/4" holes in plastic mayonaise jar lids, spaced about an inch apart, and used about a large marble worth of modeling clay around each straw. The trick is to put about the same amount of water in the jar as the length of your longest straw, screw the lid on tight so that there is no air leaking around the lid, and make sure the straws are sealed well (if you are seeing bubbles in the jar, your straws are leaking), then tip the jar so that you put the middle straw immediately into the water in the medium bowl. Don't turn the whole contraption upside down and THEN try to get it into the water...turn the middle straw down and right into the water.
Hope this helps everybody. We have had a blast with this. Thanks Science lady!