You can do a huge magic trick right in your own kitchen and be smarter than Harry Potter when you make your own yogurt. So let's do that, because when this trick works, it's like a miracle.
Here's what we're going to need: a quart of milk and 3 tablespoons of a plain yogurt that has "live cultures" (it must must says those words on the package, so look carefully, please!).
Now we're also going to need a very heavy bowl that is NOT metal or very heave not metal (not aluminum and not stainless steel) pot with a tight fitting lid. If you are using a bowl you will also need a medium sized pot that goes on the stove. Last we need some big towels to use as blankets for when the time comes to keep our yogurt warm. That's it!
Here we go! Turn the oven to 150º because we're going to need a warm bed for our yogurt to sleep in. Make sure the yogurt you are going to use is on the counter and not in the refrigerator now. Pour that quart of milk into the pot you are using, pour all of it. Now put that pot on the stove and turn on low heat. We must be very careful NOT to burn it. We have to watch it carefully, okay? Keep your eyes there and watch for the very first sign of bubbles around the edge. When you see those very first tiny bubbles, turn off the heat right away.
Okay, now we have to put our hot pot on a cool kitchen counter so it can cool down. We want it to get to about 110º. How can we know that without a thermometer? Well, 110º is just a little hotter than a hot tub or a really hot bath. It might make you say: Ouch! but it won't burn your finger or turn it red. That's how you can know. Okay?
Now when our yogurt has cooled to where we want it, we are going to get that plain yogurt with those active live cultures. They are the thingamabob that does the job, the real magic. We are going to take a plastic whisk or a wooden spoon and stir that yogurt into our milk so it's everywhere. We want those magical invisible live cultures to do their job all over the place and not just in one little corner. Okay?
And now here comes the really magical part. We put a tight lid on top of our mixed up milk. And then we wrap the whole pot or bowl in some towels so it stays warm. Warm is very important now.
Okay. Now turn off the oven heater. Wave the magic spoon over the wrapped up bowl or pot and put it into the warm oven, very carefully. Close the door. Now we have a really hard job; we have to be careful not to make the oven shake or move while our milk is sleeping in it. We need to not touch that bowl or pot for at least 12 whole hours. Then, we can take it out of the oven, unwrap those towels, take off the lid and....