Pour milk into a Dutch oven or heavy pot. Skim milk does work, but the results aren't as creamy - we recommend whole or 2% milk.

Heat the milk to 200º F (very hot, but not boiling). Stir to keep the milk from burning on the bottom. Heating the milk like this changes the protein structure so it will set properly.

Let the milk cool down to 115º F - lukewarm. If a skin forms, stir it back in before the next step.

Mix 1 cup of the warm milk into the plain store-bought yogurt (make sure the package says "live and active cultures"). Whisk out any lumps.

Combine the milk-and-yogurt mixture back in with the pot of warmed milk.

Put a lid on the pot (or cover with aluminum foil) and place the whole thing in a turned-off oven overnight. Turn the oven light on to keep things warm. You can also wrap the pot in towels - kitchen towels or even a big beach towel - to keep the heat up while the yogurt sets. Another option is setting the wrapped pot in a closed microwave next to a bottle of hot water.

Once the yogurt has reached your desired consistency, whisk it, store it in a reusable container, and refrigerate.