Re: Why do you have to punch two holes in a large juice can ?

Two holes in a juice can help the flow of fluids when you are pouring a 
glass of juice.  When you pour juice from the can, it needs to be replaced 
by air which flows into the can.  When you have only one hole, the air 
trys to get in to the can as the juice tries to get out.  The flow of the 
juice isn't smooth and it often leaps over the glass and goes all over the 
floor.

If you have a hole for the juice and another hole for the air, then the 
juice flows smoothly out of one hole into your glass, whilst the air flow 
smoothly into the can through the other hole.  

You could try a little experiment to prove this to yourself, though I 
think you should try water and not juice, and do it over the bathtub!  
Pour the water out of the can with two holes (remember that one hole must 
be above the level of the water or the air won't flow into the can), and 
then put your finger over the air hole and see what happens to the flow of 
the water.

I hope this explains it to you.