The plastic milk cartons we have hold one quart of liquid. With a nail, let's punch one hole in the side, down near the bottom of the carton. Punch another hole 3 inches up from the bottom, in line with the first hole. Then punch a third hole 3 inches above that one, so that you have three holes in the side of your carton, each above the other. Now, cover each hole with a bit of tape, making sure that the end of the tape sticks up so you can easily pull off the tape.

Now fill your cartons with water, and we'll take them outside (or, we can go outside and fill them from the hose).

Hold the cartons away from your body, with the taped holes facing to the side so you can see them, but so that when water comes out the holes, you won't get wet.

Pull the tape back from the top hole and watch how the water comes out. Now, put the tape back over the top holes and fill the cartons again.

Holding your cartons like before, pull the tape back from the middle hole, and watch how the water comes out of there. Does it make a harder stream? Does it go farther out from the carton before it goes to the ground? Put the tape back over the middle hole, and fill the cartons for the third time.

This time, pull the tape back from the bottom hole and watch the water come out. This stream should be even stronger than the one from the middle hole.

We can fit plastic tubing into the bottom holes, hold a finger over the end of it while filling the carton, then take the finger away from it and watch the water flow out from the tube. It shows a little better this way.

So. What's the big deal? What have we learned here? Have we measured anything?

Water pressure. Remember how the water from the top hole did not make as hard a stream as the water from the bottom hole? That is because the water pressure is greater from the bottom because of the weight of the water on top of it.

It works kind of like the drinking fountain. If you turn the water on a little bit, the water is thrown up a little way. If you turn it on hard, the water comes up higher. That is
because it is pushing more water through an opening the same size as that through which it pushed a small amount of water. It made harder water pressure.

Water pressure is also made by pumps. Pumps can push water up from a well and out so that it comes out gently or it comes out hard. If a lot of water has to come out fast, like the water from the fireman's hose, the pump has to work really hard and push the water out fast.

Have you ever noticed that on a hot day, late in the summer, you can't get very much water out of the hose or the faucet? Sometimes, when a lot of water is being used by a lot of people, all at the same time, the water pressure goes down, and it takes a longer time to fill your glass to get a drink, or the garden sprinkler doesn't work very well.

Sometimes when there hasn't been rain for a very long time, people who live in the country say they don't have enough water. That means that their wells are getting empty. When it rains some more, the ground water will be recharged (that means new water goes into it) and the well will fill up again.

You can see the difference in water pressure by looking at drawings of water coming out from each of the three holes. Pumps work this way also, except instead of letting the water just fall, they push the water up the pipe so that the water goes up rather than down.