Pour 1/2 cup (125 ml) of whole milk into a small saucepan. Set it on the stove over medium heat.


Allow the milk to heat to a simmer. Do not stir it during this time or allow the milk to reach a rolling boil.
As the milk heats, it will gradually separate into two parts: a liquid and a sold. This solid is made of fat, various minerals, and a protein called casein.
 Add 1 to 2 tsp (5 to 10 ml) of distilled white vinegar to the saucepan of milk. Stir until white clumps form.


After you see white clumps beginning to form, you should continue stirring for a few minutes longer.
Adding less vinegar will usually produce a slightly softer plastic, while adding more may create a slightly harder plastic.
Consider adding a pinch of cornstarch along with the vinegar. Doing so is not strictly necessary but may help the ball hold its form better. Add no more than a pinch of cornstarch, though, since too much can negatively affect the chemical reaction between the milk and vinegar.
 Turn off the heat and allow the substance to cool in the pan.


Wait until the outside of the pan feels completely cool before proceeding to the next step. This can take 30 minutes or more.
The vinegar should react with the casein during this time, producing a long chain of molecules that form a soft plastic polymer.
 Pour the cooled substance through a mesh strainer. Collect the liquid into a glass bowl and retain the solid white clumps caught by the strainer.


The liquid can be discarded once you finish separating it out.
The white solid pieces should be removed from the strainer and placed on a sheet of wax paper. These are the pieces you will use for the ball.
 Gently pat the white solid pieces with clean paper towels until they feel dry to the touch.


You will need to blot the plastic material using very light pressure. The substance will still feel somewhat tacky, and if you press the paper towel into it too firmly, it may stick and become difficult to remove.
 Gather the white pieces of soft plastic into your hands and press them together. Roll the clump between your hands until it forms a ball.


The plastic will only be flexible for several hours. This should still give you plenty of time to mold it as long as you begin immediately after straining out the solid polymer portions.
 Place the molded ball into wax paper and allow it to dry for a few hours. After that, drop the ball onto a hard surface and watch it bounce.


At this stage, the plastic should still be soft even though it can no longer be molded. The ball will be able to bounce slightly as a result.
 Return the ball to the sheet of wax paper and allow it to dry for two days or more. Try rolling the ball across a hard surface.


At this point, the plastic will be harder and may no longer bounce. The surface of the ball should no longer feel sticky, though, so you should be able to handle it without contaminating the plastic.
The ball should remain in this state permanently. If desired, you can use acrylic paint to add color or other decorations, but doing so is not necessary.
Heat the milk.
Add the vinegar.
Let cool.
Strain out the clumps.
Dry with paper towels.
Roll the substance into a ball.
Give it a bounce.
Dry until hardened.
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