I assume you guys all know that UHT Milk (ie. Ultra Heat Treated Milk) is, crudely speaking, powdered milk with tap water added back in.
For those that don't, the manufacturing process is something like this:
1. Fresh cow's milk is sprayed into a spin dryer at several hundred degC, causing the water in the milk to flash off into steam and go out a smoke stack.
2. The resulting milk solids fall out the bottom and are then either (a) mixed with filtered local tap (mains) water and packaged as UHT milk or (b) further processed to take out some of the fats and proteins that would cause the milk to go off if left in it's dry state and add stabilisers, etc. before being packaged as Milk Powder.
Why do this at all, you ask?? Bacteria cannot survive at the temperatures the spray driers operate at, giving the resulting instantly-pasteurised milk product a longer shelf life than fresh milk.
The only reason I mention this is:
1. This is why there is a label on the carton telling you to "shake well before opening" - the milk solids may have separated out if the carton's been still for a while and I'm sure no-one likes water with their cereal.
2. If in a foreign country, find out where the UHT Milk you buy was made.. You are drinking tap water from that city - not a processed white liquid from a cow!
3. Powdered Milk is a slightly more processed version of the same stuff, takes up a lot less space on a boat and at least you have control over the quality of the water you make it up with.
I hope this helps.