I used to churn butter out of fermented milk (Curd) by having the lunch bag containing the bottle of curd sit in my bicycle basket as I rode to school some 7 km away on bumpy roads.
to clarify I should have just said curd in the first sentence. However a lot of people outside of Asia or not aware. For those people “Yogurt” is the nearest thing. However the bacteria/yeast used are quite different.