With a rubberband? Used a tightly folded piece of paper, then folded in half to form a type of arrowhead. Very sting-ee.
Take the tube body of an old Bic Stic pen (just thin, white pvc) and flare the ends on the corner of something so it resembled a bazooka. Get a big, slobbery spit wad from paper towel or tissue. It hits with an authoritative SPLAT!!