The purpose of this activity is for students to estimate how much water falls on the roof of a house, given a particular amount of rainfall. For this calculation, standard units work well as the area of the roof could be given in square feet, for example, and the rain in inches. A conversion would readily give the volume in cubic feet or inches. But, the standard units used to measure volume are cups, pints, quarts, and gallons so more work would need to be done in order to figure out how many gallons, for example, there are in a cubic foot. With the metric system, liquid volume units (liters) and regular volume units (cubic centimeters) are naturally connected.

• “¿En qué momento de la actividad tuvieron que convertir unidades?” // “When did you have to convert units during the activity?” (I needed to convert meters to centimeters since the roof measurements are given in meters and the rain is given in centimeters. I needed to convert cubic centimeters to liters.)
• “¿Cómo hicieron los cálculos para hacer las conversiones?” // “How did you make the calculations for the conversions?” (The conversions required multiplying or dividing by powers of 10. I multiplied by 100 to convert meters to centimeters, and I divided by 1,000 to convert cubic centimeters to liters.)
• “¿Ustedes creen que podrían hacer un contenedor para recoger toda el agua de lluvia?” // “Do you think that you could make a container to capture all of the rainfall?” (Yes, I think that a 2-meter cube is big, but it's not so big that it would not fit near the house. Or we could use several reservoirs that are smaller.)

The purpose of this activity is for students to find out if the amount of water that falls on the house is sufficient for many of the daily household chores that use water. This task will require a lot of estimation and amounts will vary from house to house. How much of the calculations to leave up to the students is an individual teacher choice, and this lesson could easily be extended for another day if students make well reasoned estimates (some possible values are given in parentheses) for how much water is used for different activities, such as:

• Taking 1 bath or 1 shower (150 liters or 80 liters)
• Washing 1 load of clothes (100 liters)
• Washing 1 load of dishes (100 liters)
• Washing hands 1 time (1 liter)
• Flushing the toilet 1 time (10 liters)

More estimation comes into play for how often each of these activities happens and this will vary greatly depending on the student. When students make a list of the different things they do in the house that use water and then estimate how much water is used, they model with mathematics (MP4).

Consider inviting students to check their estimates by looking at one of their monthly water bills. The bill will usually give the number of gallons of water used and there are almost 4 liters in a gallon.

• “¿Cómo se pueden imaginar el volumen de agua que se usa para darse un baño?” // “How can you visualize the volume of water that it uses to take a bath?” (I can picture the bathtub filled up part way with water.)
• “¿Cómo podrían medir el volumen de agua que hay en la bañera?” // “How could you measure the volume of water in the bathtub?” (I could measure the length and width of the tub and the height of the water and multiply them.)
• “¿Les sorprende alguna de las cantidades de agua que se usan para las distintas actividades? ¿Por qué?” // “Are any of the amounts of water used for different things surprising to you? Why?” (I am surprised by how much water it takes to wash the dishes. It’s almost as much as when you take a bath.)