Did you just buy a bunch of FRUIT and then realize that one of them is bad? Are there LOADS of fruit flies around the fruit, as EVIDENCE that one must be going bad? This trap will kill any amount of fruit flies without exposing your house to harmful pesticides.... however, you probably should throw away that bundle of rotting fruit anyways....
First take off the top of the soda bottle and cut the top of a bottle (about 1/4 of the way down) so that the top bit looks like a funnel.
Next flip the top upside down and place it on top of the bottom of the bottle and tape it so it is sturdy.
Now, pour about 1/4 of an inch of juice in the bottom of the container (I used cranberry juice) and then put a couple of squirts of fruit smelling soap (I used apple soap) in the container as well.
For an additional trap you can squirt a little soap around the inside of the funnel, mainly on the bottom where the cap would be. This helps the fruit flies get stuck from just about every angle of your trap.