Using tape, attach one end of the string to of the balls.
Take the other end of the string and attach it to a table.
Repeat steps 2 and 3 with the second ball. When taping this string to the table, make sure that the balls will hang off the table two inches apart and at the same height.
Blow between the two balls.
How it happens?
Bernoulli's principle states that in areas where air moves rapidly, pressure is low. When you blow between the balls you are lowering the air pressure between the balls. Blowing between the balls drops the pressure so the higher air pressure of the surrounding air pushes the balls together.