Add 1 teaspoon of sour cream to each glass of milk. Mix well, pour in separate glasses, and place in a warm spot for 24-30 hours. Mix once again during this time. Do not cover glasses tightly. When milk has thickened, place in refrigerator and keep there until ready to serve.
Heat milk slowly, do not permit it to come to a boil. Spread one teaspoon of sour cream over sides of the glass and pour heated milk in it. Place in warm place for 8-10 hours, and when thickened keep in refrigerator until ready to serve. Do not cover glasses tightly. Air must get to the milk.