Wooden cylinder. Lid with hole. Stick with disk on end (dasher). Up-down, up-down. 20-30 minutes. Cream becomes butter.
Science
Fat globules in cream (3-5 microns) are surrounded by membrane. Churning breaks membranes. Fat clumps together. Liquid (buttermilk) separates. Rinse lump with cold water — butter.
The Amish Way
Children's work. From age 7 a child stands at the churn every morning. Not punishment — duty. By 10, they know by sound when butter's ready: shlop-shlop = too early. Thud-thud = done. No thermometer, no timer.