Pressing for explanation

In this unit students are asked to explain how and why strawberries rot over time and how and how and why yeast produce gas. Janet begins by orienting students to a “what explanation” by asking students what is happening (inside a ziplock bag full of rotting strawberries and inside the flask with yeast, starch and warm water). She then presses students to explain their ideas in small group conversation. She uses back-pocket questions (literally questions written on a card) that she has prepared ahead of time to press students for explanations and generate conversation about underlying phenomena in small groups. Then she asks students to record their ideas.

At the beginning of the unit Janet asks students to commit in writing an explanation for why strawberries decompose over time— and at the end of the unit students then write a revised story. She scaffolds these writing assignments by giving students a list of words they should include in their stories in order to increase the scientific content they include. The students also make a concept map of the key terms and their relationships as a pre-writing activity. The story-writing assignments assist both Janet and her students to assess how their explanations improve over the course of this unit.

For the yeast investigation, Janet frequently uses back-pocket questions to probe students’ explanations for both how and why yeast break down carbohydrates and produce gas. For the first round of yeast investigations she focuses students on the observable components and gets students to speculate about what they cannot see. Initially students are not given the language or the ideas to explain why a gas is formed and students tend to come up with one of two hypotheses: 1) a chemical reaction of some sort is causing gas production or 2) hot air rising causes the balloon to expand. Later students are given some “just-in-time” instruction to help the students understand how gas is produced from cellular respiration. After instruction, students investigate one of three experimental questions and develop explanations for how sugars, temperature and amount of sugar influence digestion and respiration in yeast. In the diagram below, you can see a continuum of explanations that can be “pressed for.” The first level includes “what” explanations. These are not counterproductive, however your aim as a science teacher should always be a “why” explanation that is a causal story for some important phenomenon.

Tool Systems to Support Progress toward Expert-Like Teaching by Early Career Science Educators 2008-2013 Discovery K-12 Grant National Science Foundation No. DRL-0822016