Poke a hole in the center of one piece of paper.
Insert the straw into the hole so that paper is at one end of
the straw. Hold the paper/straw combo about a centimeter above
the second piece of paper and blow through the straw.
The lower piece of paper will rise to meet the paper/straw combo.
The flow exits the straw and spreads out between the two cards. It exits at atmospheric pressure.
The exit area (circumference of the cards times the gap) is greater than the cross-sectional area closer
to the straw, therefore the air must be slowing down for mass to be conserved. This means that the
pressure must be increasing - so the air pressure between the cards is below atmospheric and therefore
the net force lifts the lower card up. In reality, the pressure on the card directly opposite the
straw will be large because of the force required to turn the flow at a right angle. This high pressure
area is small, however, compared to the area of sub-atmospheric pressure so the card still rises.