I remember those. I got them when I was in elementry school. They stopped using them because kids would pop them or accedently poke right through it. I lived in texas then. Lol I remember they made us watch a video on how to use them. Lol as if we couldn't figure out how to punch a straw into a bag.
http://wac.450f.edgecastcdn.net/80450F/kffm.com/files/2011/10/milk2.jpg
You place the bag of milk in a pitcher and cut a corner off (the cut in the picture is a little big, but you get the idea). The reason someone might buy this instead of the carton is often times the price, the quantity, but also the taste is slightly different.
http://brownhillsbob.files.wordpress.com/2011/06/milk1.jpg
When I worked in the kitchen at my Job Corps center, we had gallon bags of milk. There is a soft rubber tube at the bottom. To open it you cut the tube, and to close it you squeeze the tube in between the tines of a fork.
also, I wholeheartedly agree with your username
Ps: http://t3.gstatic.com/images?q=tbn:ANd9GcQSutsImKcdTo5B_-OwZMgq-f1VQSNVZXQsWUNWksAuhnWvzLRy2aKrOKI
http://wac.450f.edgecastcdn.net/80450F/kffm.com/files/2011/10/milk2.jpg
http://brownhillsbob.files.wordpress.com/2011/06/milk1.jpg