For this project we need to have two empty half-pint milk cartons and an arm balance scale. We will fill each carton with an equal amount of water, and hang each carton from one side of the scale. Since they have the same amount of water in them, the scale should balance perfectly. (If it does not balance, we can take a spoon or two of water out of the lower one.)

Now, we'll put a bowl under one of the cartons, and carefully poke a hole in the bottom of that carton, letting the water run out through the hole, into the bowl.

What does the scale do? Why does the scale drop lower and lower on the side with the full carton?

When we see water coming out of the faucet, or flowing down the river, or falling down in the form of rain, we don't think about it as having weight. If the sink overflows, or a river floods, would we say it is overweight and needs to go on a diet? Have you ever wondered why the kitchen sink is built into a cabinet, or what is under the bathtub? Sinks and bathtubs are pretty heavy when they are empty, but when they are full, they are VERY heavy. The cabinet in the kitchen is holding up the sink, and must be strong enough to hold it up when it is full. There are great big boards under the bathtub, because when it is full, it is terribly heavy. (Then, a person gets into the bathtub, and makes it even more heavy!)

Water is heavy; it does have weight. If we had a very sensitive scale, we could find out how much weight we gain when we drink a glass of water! All liquids have weight.
Some liquids have more weight than others.

One cup of syrup weighs more than one cup of water or milk. One cup of oil for the car weighs more than one cup of cooking oil.

We don't usually think of the weight of liquids because they have no shape, but now we see that being shapeless makes no difference to the weight of a liquid.

When we read on the side of a bottle that it contains, for example, 8 oz. of cooking oil, vinegar, milk, or oil for the car, it does not mean that it weighs eight ounces. It means that there are eight FLUID OUNCES of the liquid inside. Fluid ounces are used to measure liquids because they have no shape. There are 32 fluid ounces to a quart.

When the label on a box of laundry powder says it contains, for example, 32 oz., it does not mean that the box has a quart of laundry powder. It means that there are thirty two ounces DRY WEIGHT, of soap. Thirty two ounces dry weight is two pounds. Does one pound of water weigh more, or less, than one pound of laundry powder?