Plastic can be found almost everywhere. News about microplastics and their persistence in the environment, as well as recent reports on their effects on human health, are concerning. Plastic waste pollutes our oceans and waterways and is difficult and expensive to recycle. Ultimately, reducing our consumption of single-use plastic should be the goal, but plastic’s use in life-saving medical supplies, PPE, insulation, and shatter-proof materials means reducing our dependence on it is a challenge. Fortunately, we can all make more mindful choices about the plastics we consume.
Here is a fun STEM-based chemistry experiment you can do at home, along with a hands-on explainer on microplastics, to help kids—the scientists, policymakers, and consumers of tomorrow—understand more about plastic and its life cycle so they can make informed choices in the future to protect the planet and human health.
What Is Plastic?
The word “plastic” comes from the Greek “plassein,” meaning “to mold” or “to form.” True to their name, plastics are flexible, malleable substances called polymers. Polymers are chemical compounds or mixtures of compounds with long, repeating molecular structures, like a train made up of all the same cars. These plastics are synthetic, and today, many of them are made with petroleum, like the oils we use for fuel. But early plastics were derived from organic materials like milk protein and plant pulp.
Milk Plastic Experiment
To learn more about plastic’s properties with ingredients you can find in your kitchen, make milk plastic! Also called casein plastic, named after the protein found in dairy products, casein plastic was once used to make buttons, hair combs, jewelry, and other decorative objects, some of which are housed in museums around the world. These products were formed with a mixture of formaldehyde and milk protein, but our experiment swaps the toxic chemicals for vinegar.
This simple milk plastic uses just two ingredients: vinegar, a weak acid; and milk, which contains casein, a protein. The molecules of these substances react and create long chains. This project requires some patience. You’ll need to allow time for the milk to separate from the vinegar and for the plastic to dry once shaped.
What You’ll Need
- White vinegar
- Milk (1 or 2%)
- Measuring spoons
- Mixing spoon
- 2 heat-safe bowls or large mugs
- Microwave or stove
- Fine-mesh strainer
- Dish towels or paper towels
- Rubber band
- Silicone molds or cookie cutters (optional)
- Nontoxic paints or food coloring (optional)
How to Make It
- Measure about 4 teaspoons of vinegar into a heat-safe bowl or large mug, with plenty of room to hold about a cup of liquid.
- Heat 1 cup of milk on the stove or in a microwave safe bowl until just steaming (do not boil). Milk will be hot, so proceed with caution with little ones’ fingers for this step.
- Pour the milk into the bowl with the vinegar and stir gently as curds will begin to form. Wait a few minutes for the mixture to cool.
- While the mixture is cooling, place a strainer and another large bowl in the sink and line the strainer with a clean dish towel or layers of paper towel. Prepare a work surface at kid height that’s easy to clean. This is where the curds will be kneaded into plastic.
- When the liquid is still warm but cool enough to touch, slowly pour the liquid into the strainer, and let the curds settle as the liquid strains off. You may need to wait a few more minutes.
- When the liquid has drained out, lift the corners of the towel to create a bundle. You can place a rubber band at the top to secure the edges, or simply squeeze the towel to remove more liquid, cupping your hands to push the curds into a ball shape.
- On the work surface, knead all the curds together like a ball of dough.
- Optional: Add food coloring at this step, or wait until the plastic is dry to paint or draw on the forms.
- Use molds or cookie cutters to shape the milk plastic. Be sure to put any leftover milk plastic or curds in the compost or trash, not down the drain, as they can clog plumbing.
- Let the shaped milk plastic dry for about 48 hours or until hard and dry to the touch. Decorate!
Ideas for Discussion
- What did you notice about the texture of the curds? Did it change during kneading?
- When the milk plastic is dry, compare the plastic object you’ve made to a factory-made plastic toy. How are they different? What physical properties are the same?
- How do you think the milk plastic object will break down in the environment?
- You can repeat this experiment by asking kids to make a hypothesis about what is happening and vary the amount of vinegar, the fat percentage of the milk, or the temperature. Measuring the milk curds before kneading can help kids see how changing these variables might affect the amount of milk plastic the experiment yields.
How Does Plastic Break Down?
While milk plastic is made of natural materials that will eventually safely decompose in the environment, petroleum-based plastics break down into tiny pieces called microplastics. Some of them may be too small to see, but you can use crispy crackers as an example to show kids how microplastics break down from larger pieces to form smaller ones. (As a bonus, it serves as a valuable science lesson to show that matter doesn’t disappear, even when it’s tiny!)
To start, collect a few crackers and spread them out on a plate somewhere you don’t mind getting a little messy, and encourage kids to break the crackers apart. At each stage, ask, “What happens to the crackers?”
Continue breaking them down until the crackers are just a pile of very small crumbs. Explain how plastic, just like crackers, breaks into tiny pieces but doesn’t disappear. Instead, the pieces get smaller and smaller.
If you prefer a more outdoorsy setting, a trip to a rocky beach provides an opportunity to explain the same principle in action with sand (and, if you look closely, you might see bits of microplastic in it). While sand can be made of different materials, common tan sand is made of iron oxide and feldspar from broken-down rocks. The closer you get to the sandy shore, the smaller and smaller rocks become, until you reach the tiniest grains of sand. Invite kids to follow the path from the largest pieces of rock, farthest from the water, to the tiniest rock particles, closest to the water.
How do the rocks change in size? Ask kids to observe what happens when the waves pass over the rocks. When the waves are strong, does their placement change? Explain that over time, the water begins to smooth the rocks through erosion and weathering, tossing them onto other rocks, until the pebbles break each other down, getting smaller and smaller the closer you get to the shore, from a large rock to a tiny grain of sand you can feel between your fingers rather than easily see. When rocks decompose, or break down, the process takes thousands or even millions of years.
Plastic Pollution
A plastic bottle is just like a large rock. Over time, as it gets crushed by pressure in a landfill or by the tires of a car, or water, wind, or other environmental factors, the plastic breaks down into smaller and smaller pieces, until tiny grains of plastic are left. Instead of taking millions of years to break down, it takes from 20 to 500 years. But unlike sand, synthetic plastic is harmful to animals that ingest it, as it sometimes resembles food. Ask kids to identify some plastic items they use every day. Are there ways to use less of them? Which could be swapped for biodegradable materials?
Thinking about the magnitude of plastic pollution may be a scary topic for kids, but by demystifying the science and ecological effects with STEM-based activities, you can help them to think critically about what we consume, which may lead them to more sustainable choices in the future. For more STEM learning opportunities, sign your children up for one of STEMful’s programs and sprout their curiosity!
Footnotes:
(1) Karen Feldscher, “Microplastics Are Everywhere and Can Harm Human Health, Say Experts,” Harvard T. H. Chan School of Public Health, October 16, 2025, https://hsph.
(2) “History and Future of Plastics,” Science History Institute, accessed November 9, 2025, https://www.sciencehistory.org/education/classroom-activities/role-playing-games/case-of-plastics/history-and-future-of-plastics/.
(3) “How Does Sand Form?,” National Ocean Service, National Oceanic and Atmospheric Administration, accessed November 8, 2025, https://oceanservice.noaa.gov/facts/sand.html.
(4) “Plastic Is Forever,” United Nations, June 2021, https://www.un.org/en/exhibits/exhibit/in-images-plastic-forever.
(5) Eunju Jeong, Jin-Yong Lee, and Mostafa Redwan, “Animal Exposure to Microplastics and Health Effects: A Review,” Emerging Contaminants 10, no. 4 (December 2024): 100369, https://doi.org/10.1016/j.emcon.2024.100369.