It makes ice close and sticks together and hard to melt.
2) Why do we add the cookies and other things, only after the milk has frozen slightly?
The cookies won't move and it wasn't easy to make it hard.
3) What are two factors that affect the freezing you think?
Ice and Salt. I think also the shaking stuff
4) Why do you need a bit of air in the bag?
Because the milk can move around easy.