Fill plastic jar about half full of cream. Add one marble and seal the jar tightly. Shake the jar vigoursly in a figure-eight pattern (or close to it). When you don't hear the clank of the marble any longer, you'll know the cream has thickened. Keep shaking and listen again for the clank of the marble. When you hear that, this meands the butterfat has separated from the liquid. When you see chunks floating in the jar, stop shaking. Use a fork to scoop butter chunks into a bowl. Enjoy!
Tip: You can blend the chunks together and squeeze out excess liquid by using the back of a spoon.