Many elementary and even secondary students have simplistic views of solids, liquids and gases. They see the categories as absolute entities with distinct dividing lines. While this is not necessarily inaccurate, the notion is somewhat incomplete. Having students investigate a substance such as Oobleck (cornstarch, water, and green food coloring) can provide the experience necessary to help them see solids and liquids as more of a continuum than a dichotomy.
The lesson starts with students brainstorming all that they know about solids and liquids. After this I show several students the Oobleck in a cup tilted at an angle so the Oobleck clearly appears to be a liquid. After several students confirm the liquid nature of Oobleck, I pretend to dump the Oobleck onto a student’s head (Be sure to try this first to make sure the Oobleck is thick enough). When nothing comes out I ask, “If this is a liquid, why didn’t the student get covered in the Oobleck?”. Students sometimes suggest that the liquid was frozen, I then pour some from one cup to the next and say, “we clearly have a curious substance, I’d like you to investigate the substance in greater detail”. At this point, I might have a quick discussion about how we can organize our data or how we might go about making observations, but this can also wait until later. I then hand out small cups and pour Oobleck into the cups (Having these cups prepared with Oobleck in advanced is recommended to prevent “down time”).
After students have investigated Oobleck for a bit, we list the characteristics on the board. Some of the characteristics include: green, thick, dries out, runny, etc. We might take some time to refine these ideas by asking questions such as, “If we just said that Oobleck is green, what might other people who have not seen Oobleck think?” At some point, students mentions that the Oobleck is sometimes like a solid and sometimes like a liquid. When this comes up, I ask students to further investigate when Oobleck is a solid and when it is more like a liquid.
After some additional investigation and discussing of students’ ideas, we revisit our ideas about solids and liquids and I ask how their ideas have changed. Sometimes the students want to add a new category called a “Soquid”. However, I use this opportunity to get them to move beyond categorical thinking and introduce a line (continuum) with “solid” on one end and “liquid” on the other. I then ask where Oobleck fits. We then talk about other items and where they might fit on the line (jelly, play-doh, rocks, peanut butter, wood, etc).
Below is a quick video of a student playing with Oobleck and demonstrating its properties.