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Formative Assessment Probes

Cookie Crumbles

Using P-E-O Probes to Launch Into Activities

Science and Children—May/June 2022 (Volume 59, Issue 5)

By Page Keeley

Elementary teachers use prediction to support reading comprehension and engage students with the text they are reading. By asking students to predict what will happen next in a story, students think about what they already know from their own personal experience or prior knowledge. When students justify their predictions, it deepens their thinking and creates a desire to read further.

This reading strategy works in science as well. Asking students to predict the outcome of a science activity first before launching into the activity activates their thinking and engages students in wanting to investigate further. When students are asked to make a prediction in science, they draw upon their personal experiences or prior knowledge of science concepts to try and make sense of a phenomenon. Asking students to explain the reasoning for their prediction deepens their thinking and simultaneously provides a window for the teacher into what students understand or misconceptions that will be considered and addressed during the lesson. After students share their reasoning with each other, building upon or challenging each other’s ideas, they can test their prediction—making observations that will provide evidence to support or change their prediction. Guided by the teacher, students then construct a scientific explanation using the evidence from their observations and the scientific concepts and ideas developed through the sense-making process.

P-E-O probes can also be used with the 5E instructional model. Making a prediction to elicit students’ initial ideas and engage them in thinking about the phenomenon is the first phase in the 5E model, Engage. The next phase, Explore, is when students test their prediction, making observations. The Explain phase involves sense-making to reconcile the difference between their initial prediction and their observations or to support their initial prediction using evidence from their observation and scientific concepts and ideas developed during the lesson. In the Elaborate phase the teacher can use another related probe or questioning to see if students can transfer their learning to a different context. During the Evaluate phase, students’ revisited and revised scientific explanations of the phenomenon provide evidence of the extent to which they can use scientific practices and apply scientific concepts and ideas.

The P-E-O (Predict-Explain-Observe) probes in the Uncovering Student Ideas in Science series can be used as hands-on/minds-on activities that reveal students’ thinking and develop conceptual understanding of core ideas in science. Conservation of matter is an important core idea developed in the elementary grades. Conservation of matter begins with objects and physical changes before progressing to physical and chemical changes in substances. The “Cookie Crumbles” formative assessment probe (Figure 1) is used to uncover students’ ideas about matter conservation of an object after breaking it into pieces (Keeley 2018).

Figure 1
P-E-O Probe.

P-E-O Probe.

Students individually make a prediction and quietly record their reasons for their prediction. The teacher polls the class using the anonymous sticky bars strategy. Students commit to their prediction on a sticky note, which is then posted as a graph for a visual record of the class’s predictions (Figure 2; Keeley 2016). Recognizing that not everyone agrees on the best prediction creates a desire for students to share their thinking and work together to figure out and explain the phenomenon.

Figure 2
Prediction graph.

Prediction graph.

Students then form small groups to discuss their predictions and explain the reasoning that supports their initial prediction. During this process, some students revise their prediction when others’ reasoning makes more sense to them. The teacher then asks groups to share some of the reasons for their predictions and creates a class record of their initial thinking for the P-E part of a P-E-O probe:

  • A. The whole cookie is bigger than a broken cookie
  • A. A whole cookie is heavier than crumbs.
  • A. The whole cookie is thicker.
  • A. The whole cookie holds together more to make it heavy.
  • A. There isn’t as much cookie when it’s just pieces.
  • B. There are more pieces to weigh than a cookie by itself.
  • B. The pieces are spread out more.
  • B. There is air between the pieces when you weigh it.
  • C. It’s pieces of the whole thing so it’s the same.
  • C. It’s the same stuff, just different shapes.
  • C. It didn’t lose anything when it broke up.

As the teacher carefully listens to the students share their initial ideas without passing judgement, she makes note of students who use conservation reasoning and students who fail to recognize that all the pieces of an object weigh the same as the whole object. The class then works together to discuss how they can test their prediction, using the scientific practice of planning and carrying out an investigation, by comparing the weight of a whole cookie before and after they break it into pieces. Rather than launching into a “cookbook activity” where students merely follow the directions, the teacher guides the students into designing their own fair and accurate way to test their prediction.

Students now transition to the O phase of a P-E-O probe. Each group of students is given a cookie to weigh whole and then again in pieces. (Safety note: the cookies should not be eaten during or after the activity as they were touched by students’ hands. To avoid temptation, have extra cookies to give students during snack time or lunch.) Using a digital scale and taring the weight of the coffee filter that holds the cookie and the cookie pieces, most are surprised to find the weight stayed the same. How could that be? This leads to the most important part of a P-E-O activity—reconciling the difference between the evidence from their observation and their predictions. The teacher guides a sense-making discussion, particularly noting and involving the students who previously explained why the weight would stay the same. To formalize their understanding, the teacher now referred to the cookie material as matter and introduced the principle of conservation of matter to explain their observation. Just like an object is the sum of all its parts, the cookie is the sum of all the pieces of cookie. It’s the same amount of matter.

The students revisited the class list of ideas and narrowed it down to the reasons that supported prediction C. However, those reasons were not enough to explain what happened. They now had evidence in the form of data from their investigation they could use to construct a better explanation, now in the form of a scientific explanation. Starting with the claim that a whole cookie weighs the same as a broken-up cookie, they supported their claim with evidence from their observation that came from recording the weight of the cookie before and after. They then linked that evidence to their claim using the scientific principle of conservation of matter to explain the same amount of matter existed in both the whole cookie and the broken-up cookie. To further check on whether students could transfer their learning to other contexts, staying with conservation of matter in objects or materials and observable physical changes before progressing to substances, the teacher asked students to consider whether the weight of a clay ball would stay the same or change after flattening it or whether a piece of paper crumpled into a ball would weigh the same.

Using this simple P-E-O technique with the probes is a highly effective way to develop conceptual understanding during an activity. You can find P-E-O probes at by selecting a book. Each of the probes in the book will be listed and described, including the type of probe, such as P-E-O. In this way you can begin to develop a bank of P-E-O probe activities for lessons that address important core ideas in your elementary curriculum.


Page Keeley ( is a science education consultant and the author of the Uncovering Student Ideas in Science series (


Keeley, P. 2016. Science formative assessment: 75 practical strategies for linking assessment, instruction, and learning. Thousand Oaks, CA: Corwin Press.

Keeley, P. 2018. Uncovering student ideas in science: 25 formative assessment probes. Arlington, VA: NSTA Press.

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