Créme fraîche

8 oz heavy (whipping) cream
2 Tbl sour cream (or buttermilk)
Combine the heavy cream and sour cream in a 16 oz pyrex measuring cup. Stir well. Cover the cup with a paper towel. Allow the mixture to develop at room temperature for 8-24 hours. The bacterial culture in the sour cream will innoculate the heavy cream, converting it into a thick and tangy créme fraîche. Stir the mixture after it has achieved the desired thickness and tang. Cover tightly and refrigerate for up to 2 weeks.