Dear Mary, before we answer your question about milk, we first want to tell you something about it.
Milk is a very good food for both people and bacteria.
This kills most of the bacteria, but not all. When you open the milk at home, new bacteria from the air, from your hands, and from the glass you use can get into the milk. Normally this is OK, because you drink the milk right away. However, in your experiments, as you keep the milk, you are encouraging all kinds of bacteria to grow. You must be careful not to touch, drink or handle the samples.