Spy gear is not just for James Bond anymore, and many hobbyists have taken up the art and science of spying. You can make your own spy gear, but it is important to remember that most of the business of spying is illegal. What you learn here are things that should only be used within the letter of the law.
Cut off the headphones or earbuds from the headphones. Strip off the exterior wire coating so that the red, white and black wires beneath are revealed.
Locate the two contacts located on the photocell. Solder the red and the white wires to one of the contacts on the photocell and solder the black wire to the other contact.
Plug the jack of your headphone into the appropriate jack on your laptop computer. Plugging the headphones into any device capable of recording sound will do if you don't have a laptop.
Tape the photocell down so that it is secure and does not easily wobble or is in danger of toppling over. Use the duct tape to also attack the laser pointer to the tripod, making sure the button on the laser is kept in the "on" position so that it remains illuminated.
Aim the laser pointer at the spot where you wish to eavesdrop and place the photocell so that the beam of light from the laser bounces against the front of the photocell. Press the record button on your laptop (or other recording device) and you will be able to hear what is being said from far away.
Dissolve 1/10g of thymolphthalein in about 2 tsp of rubbing alcohol (see Resources below). Stir in 90ml of water until you get a cloudy solution.
Add about 20 drops of sodium hydroxide using the eyedropper a until the solution begins to turn into a very dark shade of blue. Stop adding drops as soon if the solution begins to get too dark or begins to look blacker than blue.
Test your disappearing ink at first by applying it to a cotton T-shirt. Disappearance of the ink on the fabric should take place in less than a minute. Writing with the disappearing ink on paper will usually take longer for the ink to disappear than applying it on fabric. Swab the area that has disappeared with cotton ball lightly coated with ammonia to make any message readable again.
Disappearing ink that is left over should be stored in a sealed container. It can be reused for up to 6 months or even a year if properly maintained.
Applying the disappearing ink to darker coloured fabrics may leave a residue.