Today I want students to think about numbers that end in zero, and what is the closet number to the number we have. (Students have used the idea of rounding, or going to the nearest Smiley Face - ends in zero- number.)
I ask students what sport season it is? (Baseball season). I ask students what shape a ball is when I draw it on the board? ( round). What shape is it in reality (when I play with it?) (a sphere). Today we will think of the picture of a ball or a round shape. What numbers have a digit that is also round like a baseball? (numbers that end in zero). Today we will talk about those numbers that end in zero and how we can use those to come up with a "ballpark" number - or estimate.
I ask, if I have 89, what is the nearest smiley face number? (90). What if I have 61? (60)
Ok, now I want you to write the smiley face numbers for the 2 numbers I say 59, and 34. Ok now I want you to add those two ballpark numbers together, and write down the ballpark answer for 59 + 34. I call on a volunteer to come up and give us the rounded (or ballpark) equation using the smiley face numbers and our ballpark estimate (60 + 30 = 90).
We try this again with several other addition and subtraction equations.
I want to give students a purpose for estimating amounts. I know that students are just beginning a unit on the environment so I think of a way to have them build a model of something that would help keep clean an area they frequent. (This could be the beach, a playground, the schoolyard, the classroom, their bedroom, etc.)
I tell students that today they will be building a "Cleaning Robot Model." They will need to think about where their robot would work and what it would need to do its work. I tell them that before they can build the robot they need to do 3 things.
First they will need to decide what their robot cleans. Next they need to draw a model of what the robot will look like. The drawing must include the sizes their robot will be in centimeters or inches. They will show the measurements for what they imagine their real robot would be. They can estimate the sizes, but then use actual measurements to determine if the sizes they have chosen make sense.
2MD.A3 expects that students will be able to estimate lengths in feet and inches and centimeters and meters. When students are thinking about the size of their robot and its parts, they are estimating the size based on what they know of inches, and feet, or centimeters and meters. I am looking to see if they can be logical in their measurement estimates.
Finally they will need to choose and pay for materials I will present. I display construction paper ($0.69/piece) , cardboard (1.79), wood (3.64), aluminum foil ($0.78/foot), string (0.69/foot), bottle caps ($0.38), pipe cleaners (2.45/3) and recycle bottles ($4.87). They will need to see if they have enough money to do what they want to build. I tell students that they are starting with $50.00. They should find the smiley face numbers for each price I have displayed and then add those up to get a ballpark estimate of whether they have enough money or not. I remind them that if they need more than one of something, they would have to list each one as a separate cost.
I do not have the students add up the actual costs manually as for some students that would mean adding a column of 3 digit numbers. I tell students to use a calculator to add the exact prices and compare it to their estimated total. I remind them to enter the decimal point as we have learned in the past. We discuss how the 2 numbers should be close to one another (in this case I suggest that they be within $5.00 due to the option to round up or down.) Using the calculator helps students practice putting in the decimal point when adding money and it allows for validation of their estimations.
I hand out the design sheet, materials page and a shopping list page. I explain each page to the students. Now I give them about 35 minutes to fill in these page. I do tell them that they can use the first 15 minutes to draw the robot, but that the other 20 will be for measurements and pricing of materials.
I answer any questions (such as when are we going to build the real models (it is important to find 45 minutes for building by either extending the math block for a day or using a different block - I used a science block,) and then let students get to work.
Students will be using mental math today as they add the smiley face numbers. Many students will do this mentally which reinforces 2NBT.B8 (mentally adding 10 or 100 to a given number).
Building the robot is the most exciting part of this lesson. I have used the lists to gather supplies to sell to students. They bring their list and I give them the materials. They are then free to create the robot that they have designed.
I take the time to have them pay with the $50.00 and help me count their change. This way if they need additional items, they know how much they can buy. This reinforces the making change lesson students recently completed, and it helps students with place value counting that is a strong part of the Common Core standards for second grade.
Allow students plenty of time to do build their robots. (Extending the math block, or adding a separate block for the building may be necessary here) I made sure that if students wanted additional materials that they would have to buy them. I had them check to make sure that they didn't go over their budget. Some students traded materials with other students rather than spend their money.
I asked students to attend to the sizes or measurements they had set for their robots. MP6 suggests that students should attend to precision, and I wanted to observe their ability to do this. (If they wrote 3 inches for an arm and measured 4, I did not make them do it again, but if they wrote 3 and then cut a piece 3 feet long, I questioned if they were using the correct measurements and trying to build their robot to the sizes they had set up.) I also let students choose their own measurement tools. I laid out rulers (6 and 12 inch), centimeter rulers, yard sticks, meter sticks and tape measures. MP5 suggests that students select and use tools correctly and I wanted to reinforce that standard.
Each robot is a unique creature designed to clean up a different habitat. They lend themselves to story writing because of how different they are, and the fact that students have imagined how their robots might work.