Shake a jar of cream until it turns into butter

Buy a carton of the highest-fat cream you can find, pour half a jar, lid on, and shake. For most of ten minutes nothing happens — then it's whipped cream, then one shake later it thuds and splits into a lump of butter sloshing in buttermilk, a turn you cannot picture until it happens in your hands. Salt it, toast something, and eat it while your arms file a complaint.