• Milk
• Yogurt starter (previous batch, plain store-bought yogurt, or fancy yogurt starter bacteria)

Heat milk on stove until it is steaming, but not boiling. Pour milk into glass container(s). Preheat oven to warm. Let the milk sit until it is cool enough to keep your finger in it for 10 seconds. (Make sure your finger is clean!) Mix in yogurt starter. A couple tablespoons for about 3-4 cups of milk seems to work pretty well. Turn off the oven, wrap the glass container(s) in a towel, and place in the oven. Let it sit for 8-10 hours.

For thicker (Greek-style) yogurt, strain the yogurt for several hours.