Digestion - Why Do We Eat?

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Students will be able to create an evidence based explanation to answer, “Why do we eat food?”

Big Idea

After collecting temperature data and reading texts on calories, students use Claim, Evidence, and Reasoning to answer "Why do we eat food?"


5 minutes

The focus of this lesson is not the digestion process, but rather to develop an evidence based explanation to answer the question, "Why we need to eat?".  This lesson can be done either before or after you have covered the digestive system.  It also would work in a nutrition unit for those who teach human health.  

To engage students in the lesson we review the human digestive system using the video Human Digestive System, by Apollo Hospitals Dhaka.  

Next, I show This is 200 Calories (by ASAP Science). It introduces students to the concept of calories. Understanding what calories are is important to understand, both for this lesson and in everyday life as "calories", like energy, is one of the commonly used but little understood words.

As students watch this video, they are expected to listen for and record the answers to the following questions. You will want to circulate during the videos to help remind students that this isn't time to "turn off" attention.

1)  According to video calories are simply units of ______________.

2)  What's the definition of a calorie?

3)  Complete the following:

Fats : _________ cal/g

Carbohydrates: __________ cal/g

Proteins: _________ cal/g

4)  What can you infer about the composition of a big mac knowing that only a a quarter of a big mac has the same amount of calories as a whole platter of sliced apples? Hint:  Look at #3.

5)  How many calories does an average adult need daily?


20 minutes

In the next section of lesson students begin to explore the concept of calorie as a measurement of energy.  In this activity students use a homemade calorimeter (SP2 -Develop a model to describe unobservable mechanisms). 

A particular food item will be ignited, the homemade calorimeter will trap the heat of the burning food, and the water above will absorb the heat, and cause the temperature (T) of the water to increase. By measuring the change in temperature (ΔT) of a known volume of water, students will be able to calculate the amount of energy in the food tested because the heat gained by the water will equal the heat lost by the food item. (MS-PS3-5. When the kinetic energy of an object changes, energy is transferred to or from the object, Crosscutting Concept - Energy and Matter -The transfer of energy can be tracked as energy flows through a designed or natural system.)

Remember: A calorie is the amount of energy, or heat, it takes to raise the temperature of 1 gram of water 1 degree Celsius (1.8 degrees Fahrenheit). 

Advance Prep for Lab:

  1. Cut soda cans in half, cover edges with masking tape.
  2. Punch holes in sides, thread pipe cleaner through to create a handle.

Before we start lab I have students answer, "Why do we need to eat?" on the Claim Evidence Reasoning handout.  

Some possible sentence starters can be 1) We need to eat because ..... 2) Food gives us .....

I have included a presentation about calories, and why we eat, which also contains a visual of the lab set up and the data analysis table.


  1.  Soda cans
  2.  Water (50ml)
  3.  Beakers
  4.  Thermometers
  5.  Pecans
  6.  Cheerios
  7.  Paper Clips
  8.  Clay
  9.  Masking Tape
  10.  Pipe Cleaners


  1. Distribute materials to each group and have them set up their station and get water.  Students should take the temperature of their water and record the temperature in Lab Data
  2. Give each group a Cheerio to put on their paperclip, then come around to light the Cheerio on fire. Students should hold their can of water over the flame until the flame burns out, then take the temperature of the water and record it in Lab Data.
  3. Repeat step 2 using the pecan.
  4. Help students to calculate the change in the temperature of the water in Lab Data.

  1. Have students answer the following questions in their notebooks:
    • Which food item increased the temperature of the water the most? What does this show?
    • What food item would you choose if you were looking for a quick burst of energy?  Why?


15 minutes

In this section of lesson we continue to learn more using three readings on Calories.  To help students in reading these science texts, I use the clarify strategy from Writing in the Margins.  

This strategy clarifies complex ideas presented in the text. Readers clarify ideas through a process of analysis, synthesis and evaluation. Pausing to clarify ideas will increase your understanding of the ideas in the text, particularly Reading 3 which addresses photosynthesis and cellular respiration.

In order to clarify information you might...

  • Define key terms.

  • Reread sections of the text.

  • Analyze or connect ideas in the text.

  • Paraphrase or summarize ideas. 


5 minutes

In this section of lesson students answer the guiding question of the lesson "Why Do We Eat"? Students are required to develop a scientific argument with the inclusion of a claim, evidence, and reasoning.  

I scaffold this through the use Claim Evidence Reasoning which is a graphic organizer for the development of an argument through the use of claims, evidence, and reasoning.  

1)  To aid students in finding evidence in readings students complete MIP's (Most Important Points) for each individual reading (RST.6-8.1 - Cite specific textual evidence to support analysis of science and technical texts, attending to the precise details of explanations or descriptions.)  MIP's are anything that students feel are important, interesting, and thought-provoking.

2)  After students have completed MIP's student write down a claim to the guiding question.

3)  Students are then required to write down evidence from reading (including source) and experiment.

4)  Last step is for students to write down reasoning for each piece of evidence (both from readings and experiment) .

5)  Students will use this graphic organizer to complete exit slip in the evaluate portion of lesson.


5 minutes

In this section of lesson students complete an Exit Slip that requires them to develop an argument through the use of claims, evidence, and reasoning (W.7.1 -Write arguments to support claims with clear reasons and relevant evidence & MS-LS1-7 Develop a model to describe how food is rearranged through chemical reactions forming new molecules that support growth and/or release energy as this matter moves through an organism).  Students are encouraged to use Claim Evidence Reasoning to aid them in developing their argument.  

Since the majority of my students have difficulty in connecting their ideas I provide them with Transition Words, which provides them with examples of transition words for the following:

1) Agreement/addition

2) Cause/condition/purpose

3) Effect/consequence/result

4) Examples/support/emphasis

5) Conclusion/summary/restatement