B4 Casein milk plastic
Theory
When a new compound is formed in solution it often can be filtered out.
When adding an acid (citric acid or acetic acid) to milk, the proteins in the milk precipitates as into a solid, that can be filtered out. This protein is called casein.
What do you need?
Coffee filters, cups, milk, citric acid or acidic acid (vinegar)
What do you have to do?
Take ~100 mL of normal milk and add a teaspoon of citric acid (or 3 teaspoons of acidic acid).
Stir the mixture until the mixture gets viscous. (If it doesn't get viscous add a bit more acid).
Then place a coffee filter over a cup and pour the viscous milk into the coffee filter.
A white substance should remain in the filter and an almost transparent liquid can be collected in the cup.
You can pour the white substance into a form and let it dry eventually yielding a white solid.
Additional theory
The white solid consists mostly of the milk protein casein.
This can be further processed into the plastic called Galalith, which was used as a substitute material for horn tissue. It is still sometimes used for buttons.
If you want to learn more about plastics, especially rubber, follow this link:
https://www.geogebra.org/m/zf5vt822