Remove the soda cap. Then, flip the top of the bottle upside down (so that it looks like a funnel), and slide it into the bottom portion of the bottle to complete your fly trap.
How to Bait Your Trap:
Fly larvae feed on rotten meat and feces, while adult flies feed on sweet things like decomposing fruit. Bait your trap with either of these things, and you'll attract lots of flies. To keep bees out of your trap, add a splash of vinegar to the mix.
A Note About Wasps: Meat and sweets will also attract wasps to your trap. If you only want to catch flies, bait your trap with fruit and vinegar in the early spring and fall months. Switch to meat and vinegar in the summer.
Want to get rid of wasps too? This trap can serve a double purpose. Just use meat and vinegar as your bait in the early spring and fall, and sweets in the summer.
How the Trap Works Flies smell the bait, and fly into the bottle to get to it (either to lay eggs or to feed). Once inside they aren't able to navigate back up the narrow opening, and they become trapped.
Maintaining Your Trap:
Empty the dead flies and re-bait your trap regularly. If you decide to use meat or animal droppings (chicken litter, rabbit pellets, etc.) as bait, you'll also need to destroy any larvae that you find in your trap. Filling the bottle with hot water should do the trick.