Re: why do drinking boxes collapse when we drink from them?

Good observation, Ashley.  Drink/juice boxes don't collapse all the time 
but here is what is happening when they do:

Drink/juice boxes are designed to have a hole punched through a 
foil/plastic circle with a small straw for no-spill drinking.  Often, a 
tight seal is made between the foil and the side of the straw (so there 
are no gaps between the two), which prevents air from getting back into 
the box as you suck the drink/juice out of it.  The fact that liquid is 
being removed from the box but nothing is going back into the box to 
replace that juice volume, creates a vacuum inside the box.  When the 
vacuum is strong enough (e.g. you've removed most of the juice all in one 
session), the sides of the box will collapse.

Why?  Because the pressure of the air outside the box overcomes the 
strength of the box walls.  The force exerted by the outside air on the 
box walls is much greater than the combined laminate (material the box is 
made of) strength and small amount of air and liquid left inside the box, 
and the box walls collapse inward... it's the same principle as if you 
squeezed the sides of a Pepsi can.  When the can is closed, there is 
enough equal and opposite pressure inside the can (working against you) 
that you can't collapse the can.  When the can is empty and you squeeze 
hard on the sides, the can walls will cave in.  In the case of the juice 
box, you are creating a void inside the box, which allows AIR outside the 
box do the squeezing for you.

I hope this explanation isn't too confusing.  Enjoy!

Kieran