Great activity for kids on a rainy day.
A Jar with a lid
1. Fill a jar halfway with heavy cream (baby food jars work great for small batches), and screw the lid on tight.
2. Shake the jar up and down until the cream thickens and begins to stick together.
3. Then, open the jar, and pour any remaining liquid into another container; this is the buttermilk. Everything else is butter.
4. Knead the butter under cold running water for several minutes to work out any remaining buttermilk (otherwise the butter will spoil quickly).
5. Knead in salt, if desired.