Take a jam jar with a screw-top lid. Make two small holes in the lid, and push a plastic straw (straw A) through one of the holes, so that about 7cm has gone through. This straw will be the basis of your fountain. Push a separate plastic straw through the second hole (straw B) but make sure that only a couple of centimetres appear on the other side. Make sure the holes around the straws are watertight using Plasticine or clay. Put about 5cm of coloured water into the jar. Put the top on the jar, once again making sure that the seal is watertight.
Take a second jam jar and half fill it with coloured water, perhaps using a different colour. This is the water that will make the fountain.
Set up the experiment by turning the first, lidded, jam jar upside down over the second, open-topped jar. Place the end of straw A in the water in the second jar, and the end of straw B over a bucket. Use a clamp to hold the jars in position and watch what happens. You should find that water spurts into the upper jam jar like a fountain.
Experiment with the fountain by using different sized straws. The water in the top jam jar exits through straw B. As this reduces the amount of water in the jar and as no more air can get in, the air pressure in the jar is also reduced. The air pressure outside the jar is greater than the air pressure inside the jar, and water is forced up through straw A.