Opening a brand new carton of juice can be disastrous if you're not careful. The air pressure on the inside of the carton is waiting to exit, but does so in a vigorous force when opening a carton for the first time.
This causes the liquid to force out in a violent wave, which can potentially spill all over the place—everywhere besides the intended cup.
The solution to this problem is easy—poke a hole in it.
Poking an air hole on top allows the force from the inside of the carton to be released from two different areas, thus relaxing the exertion of the liquid.
This tip will also work with plastic bottles too, like a milk jug.