To make your own butter, you will need some well-chilled heavy cream, and some small containers with screw on lids. Baby food jars work well for the youngest butter-makers, or you can use a canning jar for older kids. The larger your jar, the longer it will take to make your butter. Just make sure that whatever container you choose has a screw-on lid as the gasses that are released as the butter forms may cause the lid of a pop-on lid to literally shoot right off of the container.
Fill your container about 2/3 of the way full with the cream. Now, shake, shake, shake the jar. You may have to help out because it will take quite a bit of very vigorous shaking to make butter. Every once in a while, remove the lid to observe what is happening inside. At one point you will see that the cream has become fluffy as it has turned into whipped cream, but it won't taste like whipped cream, because you have not added sugar to it. Eventually it will sound as if the whipped cream has turned back to a liquid, and when you remove the lid you will see that a ball of butter has formed, leaving behind a thin liquid, which is buttermilk.
Once all of your shaking has yielded a ball of butter, drain off the buttermilk and remove the butter to a small dish.
The Science Behind It
Cream is an emulsion that has minute droplets of fat dispersed in water. Shaking the cream makes the fat droplets stick together, forming butter. An emulsion is a type of colloid: it has tiny particles of one substance scattered through another. A colloid is a type of mixture: two or more substances jumbled together but not chemically combined.