Lesson plan Grade 2 Math
Two Pans at Otterbank: A Grade 2 Evaporation Measuring Lesson
Students measure water depth in an open pan and a covered pan with a centimeter ruler, then subtract to find how much water left each pan in a week.
Objectives
- Students will choose and use a centimeter ruler to measure the depth of water in a pan.
- Students will subtract two depths in centimeters to find how much the water level dropped.
- Students will compare the drop in a covered pan with the drop in an open pan.
Vocabulary
Key words: depth, centimeter, evaporate, covered, compare
Teacher overview
Two identical pans of water sit on a windowsill for a school week, one left open and one sealed with plastic wrap, and students measure the depth in each pan every morning with a centimeter ruler. The measurements give a clean subtraction context while the covered pan acts as a control that rules out a leak. The misconception this lesson targets is the common grade 2 explanation that the missing water soaked into the pan or leaked out the bottom, which the covered pan disproves because drops collect on the underside of its wrap instead of disappearing. A second error worth watching is starting the count at the one mark instead of the zero mark, which makes every depth one centimeter too large.
Procedure
Hook (5 min): Pour water into a clear cup and mark the level with tape. Ask students to predict what the level will look like on Friday if the cup stays on the windowsill. Record two or three predictions on the board and tell the class that a group at the school in the town of Otterbank ran this same test last spring.
Set up and model measuring (12 min): Fill both pans to the same depth and cover one with plastic wrap. Hold up a yardstick and a centimeter ruler and ask which tool fits inside a pan this shallow. Model standing the ruler on the bottom of the pan with the zero mark touching the pan, then reading the number at the water line. Read one depth incorrectly on purpose by starting at the one mark, and ask students to say what went wrong.
Students measure and record (10 min): In groups of four, students take turns measuring the depth in each pan and writing the whole number of centimeters on a group chart. Groups compare their two readings for the same pan and remeasure whenever the readings differ by more than one centimeter.
Handout work (18 min): Distribute the handout with the recorded measurements from the class in the town of Otterbank. Students work the six problems, drawing a number line or a labeled ruler sketch for any subtraction they cannot do mentally. Circulate and ask two students to explain problem 3, since comparing one drop with another drop is a second layer of subtraction that many students collapse into a single step.
Closure (5 min): Ask the class to answer one question with a show of hands: did the covered pan lose more water, less water, or the same amount as the open pan? Expect at least three quarters of the class to answer less. If fewer do, revisit the two Friday depths side by side before dismissal.
Student Handout
Imagine you are the class at the school in the town of Otterbank. Here are the depths your class measured. Pan A was open. Pan B was covered with plastic wrap.
| Day | Pan A depth | Pan B depth |
|---|---|---|
| Monday | 8 cm | 8 cm |
| Tuesday | 7 cm | 8 cm |
| Wednesday | 6 cm | 8 cm |
| Thursday | 5 cm | 7 cm |
| Friday | 4 cm | 7 cm |
- Pan A was 8 cm deep on Monday and 4 cm deep on Friday. How many centimeters did the water drop?
- Pan B was 8 cm deep on Monday and 7 cm deep on Friday. How many centimeters did the water drop?
- Pan A dropped 4 cm. Pan B dropped 1 cm. How many more centimeters did Pan A drop than Pan B?
- Pan A was 7 cm deep on Tuesday and 6 cm deep on Wednesday. How much did it drop in that one day?
- On Friday, Pan A was 4 cm deep and Pan B was 7 cm deep. What is the total when you add the two depths?
- Suppose Pan A keeps dropping 1 cm every day after Friday. Pan A was 4 cm deep on Friday. How deep will Pan A be after 3 more days?
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Answer Key
- Answer: 4 cm. Subtract the Friday depth from the Monday depth: 8 - 4 = 4 centimeters.
- Answer: 1 cm. Subtract the Friday depth from the Monday depth: 8 - 7 = 1 centimeter.
- Answer: 3 cm. This compares the two drops from problems 1 and 2: 4 - 1 = 3 centimeters. A student who answers 4 has repeated the Pan A drop instead of comparing it with Pan B.
- Answer: 1 cm. Subtract the two depths for those two days: 7 - 6 = 1 centimeter.
- Answer: 11 cm. Add the two Friday depths: 4 + 7 = 11 centimeters.
- Answer: 1 cm. Take away 1 centimeter for each of the 3 days: 4 - 1 - 1 - 1 = 1 centimeter.
Counting back from 4 on a number line is an acceptable method.
Differentiation
Support
Give these students a paper strip already marked in centimeters that they can lay beside the pan instead of standing a ruler upright, and start them at problems 1, 2, and 4, which are single subtractions with small numbers. A number line from 0 to 10 taped to the desk lets them count back rather than subtract in their heads.
Extension
Have students predict the Monday depth of Pan B for the following week and write a sentence defending the prediction using the pattern in the table. Ask them to explain where the drops on the plastic wrap came from and why the covered pan lost much less water than the open pan, and discuss whether plastic wrap seals a pan perfectly.
ELL supports
Demonstrate the words "open" and "covered" with the two pans in hand before any reading. Provide a sentence frame for the closure discussion: "Pan ___ lost more water because it was ___." Post a labeled picture of a ruler with an arrow at the zero mark so the measuring instruction does not depend on English alone.
Extensions
Repeat the setup with one pan on the windowsill and one pan in a shaded corner of the room, keeping both uncovered, and have students measure for another week. Comparing the two new drops gives a second subtraction data set and introduces the idea that sunlight changes how fast water leaves an open pan. Students in the town of Otterbank could graph both weeks side by side once they have worked with bar graphs.