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Use a craft blade to cut the bottle first, then trim off any jagged edges with scissors. How far down the spray bottle you cut is up to you; it needs to be far enough down so that it is comfortable to hold. About 3 to 4 inches (7.6 to 10.2 cm) below the cap would be ideal. Discard the bottom part of the spray bottle.  If your spray bottle has a molded handle, cut just below the molded handle. If you can't find a spray bottle, use a small air pump instead, like the kind used to fill basketballs. Be sure to take the unscrew the top nozzle.  Wrap some colored tape around the bottom, cut edge of the spray bottle. This will add a little design, and hide any jagged edges. After cutting your spray bottle, you will likely have a plastic tube sticking out from under the cut edge. This will only get in the way, so trim the tube with scissors until it is the same length as your cut bottle. You don't want anything sticking out from under the cut edge. You can cut the tube a little bit shorter, if you want to. This will prevent it from getting caught on anything. The hole should be big enough for the nozzle of the spray bottle to fit though. You can drill the hole with an electric drill, a dermal, or even a pair of scissors.  Make sure that your bottle has a sports cap. They are the kind that you pull to open, and push to close. Don't use a standard water bottle with a flat, twist-on bottle cap. Twist the nozzle on the spray bottle until it is in the "open" position. Draw a ring of hot glue around the nozzle, then push the nozzle into the hole. Wait for the glue to set, then add more glue around the seam. Adding more glue around the seam is a must. You want it to be airtight. If it is not airtight, there won't be enough pressure, and the air gun won't work. Pull the cap open first, then place a straw right over the opening. Draw a line of hot glue around the seam. Be careful not to get any glue inside the straw, or you may block the hole. You need to be able to push and pull the cap close/open. If you want something sturdier, you can use a short plastic or metal tube instead. You can also take a ballpoint pen apart, then use the body.
Cut a spray bottle in half to make the handle. Trim the tube inside the spray bottle until it is level with the cut edge. Drill a small hole in the bottom of a plastic water bottle. Open the the spray bottle's nozzle, then glue it into the hole. Glue a straw on top of the sports bottle cap.