Microwave 1 cup of milk for 1 minute and 30 seconds, or until hot, you do not want it boiling.
Then stir in 4 tablespoons of vinegar into the milk.
If you want to, you can add food coloring to the milk and vinegar while you're stirring .
Keep stirring the milk and vinegar; the milk should start to curdle. Stir for about 1 minute.
Strain the milk and vinegar to get all the liquid out.
Transfer the plastic milk to a paper towel and squeeze out any more liquid.
You can then shape it and let it dry for two days to have it keep its shape.
Science Behind It
This "plastic" you made is actually called casein. This is created when the milk meets the vinegar because the casein protein in the milk does not mix with the acid in the vinegar, therefore the casein clumps together to form blobs. This isn't a true plastic because plastic is made with polymerases.
Tips
This could be a good project to do with your art project to tie in science to other 4-H projects. You can also do this project in the winter if you want to make your plastic milk into holiday ornaments. For older members you can try to experiment with different amounts vinegar to see if you get more or less plastic. They can also experiment with different types of milk.