This blow dart & gun is about as simple as you can get... All you need is an old shoe lace, a bic pen and a straight pin. They are fun and work really well, but can also cause damage and injury. This definitely qualifies as a DFO - or Dangerous Flying Object.

Step 1: Prepare the Blow Dart

To construct the dart, take the old shoe lace and cut it about an inch behind the plastic end piece. Then using a straight pin pull apart the lace into a bushy tuft of strings. Take a look at the photos to see how it should look when fluffed out sufficiently.
Next, trim the plastic tip as needed... it should be shorter than the pin to be used.
Once trimmed, push the pin from the tuft of lace through the plastic tube until it protrudes out the other end.

Step 2: Prepare the Blow Gun Tube

To construct the blow gun, dismantle the ball point pen (a Bic works great). Remove the cap, the ink tube, collar and end plug... these parts can be set aside. Keeping the pen tube... you now have your blow gun.

Step 3: Loading & Shooting

Now take your finished blow dart and push it into the pen tube. Sometimes it helps to twist the dart as you push it into the tube. Once the dart is fully inserted into the tube, place your lips around that same end of the tube and blow... making sure nothing important is directly in front of the tube. Be careful, the blow darts can easily be shot across the room. And the darts may stick in walls, doors, skin, eyeballs, etc...

Step 4: Extra Info

The plastic tips of a shoelace makes the easiest dart, but do not throw away the rest of the shoelace... with a little duct tape, you can create a vast arsenal of darts from a single shoelace. Cut the remaining shoelace into 1.5 inch segments (or just a bit longer than the pin to be used). Take a short piece of duct tape - about half an inch wide and a couple inches long (or roughly 1.5 x 5 cm). Wrap the duct tape tightly around one end of the shoelace segment. Trim the duct tape tip to about half the length of the pin. Follow step 1 to fluff out the shoelace and then insert the pin through the duct tape tip.
Ideally, the dart should fit snug in the tube. Too loose and the dart will not shoot with much velocity. Too tight and you'll go blue in the face trying to blow the dart out. A nice snug fit will ensure the best performance of your simple blow gun. Enjoy!