This project will show us a little bit about the weight of water. As we discussed before, everything has weight; even spider webs and pencil marks. We often do not think about some substances in terms of weight. That is probably more so with liquids than with anything else. The reason might be that liquids are not measured by weight. Liquids are measured by volume, and stated as ounces, cups, gallons, etc.

We wouldn't go to the store and ask for a pound of milk! That brings up an interesting point. When we buy cream, we buy a half-pint or a pint, or even a quart, of cream. When we buy ice cream, we buy a pint, quart, half-gallon, or gallon. Well, there is something else made of cream that we buy by the pound. Can you think what that is? Can you think what the reason might be?

Anyway, we have this little scale that is used for postage. It is quite accurate, and can be used for weighing small things. First, we will weigh a washcloth, and make a note of its weight. Now, we will dip the washcloth into water and squeeze out as much water as we can. Let's weigh the wet washcloth. Wow! It weighs a lot more, doesn't it? Probably has the same weight now as maybe ten dry cloths. Next, we'll dip the cloth into the water again, and not squeeze it--just let the cloth keep as much water as it can hold. That REALLY weighs a lot! It must be the water that causes the weight.

We have three small milk cartons saved from the cafeteria. Each one holds the same amount. Let's fill each one with a different liquid, and see if they all weigh the same.

Water can be one liquid, we'll use cooking oil for another, and liquid dishwashing soap for the third. All three substances in the cartons fall into the category of "liquid;"
there is exactly the same amount in each carton. Therefore, they should all weigh the same. Right?

Wrong. Try to estimate which one weighs the most.

("Estimate" is a fancy word for "guess." The difference is that when we make an estimate, we are supposed to know a little bit about the thing we are estimating. We can guess at stuff without knowing anything. It just isn't any fun!)

Why would some liquids weigh more than others?

Have you thought of what it is that we buy in liquid measures when it is cream, but buy a pound of it when it has been made into something else?

Butter!