Subject: Why two Identical tapered cups placed one above other,is hard to seperate?

When two identical kitchen vessels, for example, Kept one above the other and 
pressed, needs a heavy amount of force to seperate them...Is it because of the 
vaccum that is created between the gap of the vessels or due to any other 
factor???...If vaccum pressure is the only reason, what are the governing 
parameters that indicates the necessary force required to seperate 
them?....Thankyou for ur patience........

Re: Why two Identical tapered cups placed one above other,is hard to seperate?