Pour a little into a glass, or just remove the cap from a bottle. (It doesn't have to be full — nearly empty will also work.) Cover the opening in plastic wrap and secure with a rubber band. Then, poke a few small holes for the fruit flies to enter. They can't resist the scent of vinegar, and they won't be able to exit once they're inside. For an even better chance at success, make several of these DIY traps and place them around your kitchen.