Cut a circle out of clear contact paper (sticky on one
side - usually used to cover books)- bigger than the
opening of the cup.
Cut small slits in the edges so as to make the circle fit
properly onto the cup and put over mouth of cup so it is
sealed. Also put a rubber band around the top to keep contact
in place
Pierce a hole in the bottom of the cup and about 12 small
holes in the contact.
Dip the contact into a detergent solution.
Blow through the bottom of the cup and watch the bubbles
come out the other end.