Look out! It's an octopus!
Use a screw to drill a hole near the top of a clean, de-labeled soda bottle, and another near the bottom. Fill the bottle halfway with water, replace the cap, and you're ready to perform magic. When you cover the top hole with a finger, nothing flows out of the bottom. Remove your finger, and out pours the water. At our testing, 5-year-old Lila played with this for half an hour, and later announced, "I'm going to be a scientist when I grow up, because I like to figure things out."

Now add more holes. Our kids loved watching air bubbles float upward when the bottle was submerged. Once it was filled with water, we pointed out how the holes, while all the same size, produce different-sized streams, and how the streams change as the water level decreases in this bottle we called "the leaky boat." (Watching streams of water flying in all directions, like tentacles, Danielle, 5, yelled, "It's an octopus!" A much better name.)
Ask the kids... how the water flow will change if you block different holes or add holes.
What's going on? Water can't escape from the bottle unless something else (here, air) can take its place. With multiple holes of the same size, greater pressure makes bigger streams, such as at the bottom of the bottle, where there's more weight from water.