Unscrew the two halves, and take a look inside.  Take out the plunger/barrel assembly and spread it all out.  See the piece closest to the plunger casing?  Take that and drill a hole straight through the center, making sure that the hole is smaller than a crayola barrel (the thin end).  Now, take the thin end of the crayola barrel and hot glue it to this piece, making it a straight and perfectly in-line with the hole.  Now, hot glue this piece back into the plunger casing.  You can get rid of the barrel and the other piece that had the barrel stem on it.  Tape over your hot gluing job on the barrel and the plunger casing.  Put it all back together (don't forget to stretch that spring!)  Rubber band it and you're done!