Usually I'm blogging about astronomy, but the March issue of Science and Children had an article "Water Pressure in Depth" that covers an experiment that is part of the Webelos scientist activity pin, and something I have done with Webelos. They turn the simple demonstration activity into an inquiry activity. They also use plastic milk cartons which are easier to come by than the coffee cans I used (doh! why didn't I think of that then).
To start off the activity, you take a gallon jug and a 1/2 gallon jug and poke holes in them. On the gallon jug, poke the hole low, near the bottom, and in the 1/2 gallon jug poke the hole high, near the top (but not too high - remember you still have to cover it with a couple of inches worth of water). You can cover the holes with tape; masking, duct or packing, whatever works well. Fill both with water to the same level; the gallon jug will have water well above the point of the hole and the 1/2 gallon jug will be much lower.
When you remove the tape (oh, you did remember to use a catch basin so you don't have a mess on the floor, right?), the water from the gallon jug will shoot out much further than that from the 1/2 gallon jug. The question is: is it because there is more water in the gallon jug or because of the height of the water above the hole?
That's the whole point: get the kids to start asking why and then design a couple of experiments to decouple the variables.
Hint 1: you can poke the holes at the same height on both jugs, either low or high, and see what happens. This isolates the volume of the jug as a variable.
Hint 2: you can poke holes in two jugs the same size (both 1/2 gallon or both gallon) at different heights and see what happens. This isolates the height of the water above the hole as a variable.