Place 1 cup heavy cream and 1 tablespoon freshly squeezed lemon juice or distilled white vinegar in a clean 1 pint glass jar and stir with a clean spoon until combined. Cover with a paper towel or cheesecloth and secure with a rubber band. Let sit at room temperature ( 72°F to 78°F) for 24 hours until thickened and tangy. Stir well, cover with the lid, and refrigerate for up to 2 weeks. The sour cream will thicken more upon refrigeration.