You can use leftover yogurt as a lead and add it to milk to make yogurt. However, it's best to do it 2-3 times as a good idea, and then use new yogurt or mushroom powder as a primer to make new yogurt. If you don't have yogurt, you can buy mushroom powder on Taobao, which is usually more than ten dollars for a big packet with ten small packets, and each small packet can make 1000 grams of yogurt. After the first time to do out, you can eat the leftover yogurt as a primer, do the second and third time.