Using only a rubber band and permanent marker or ink pen, it is possible to encrypt and decrypt a text message using only the muscle power in your fingers.
Step 1: Let's stretch before begining
Take a rubber band that is wide enough to write on, and stretch it tightly around a flat surface. The surface could be a book, or in this case, a roll of duct tape. The tighter the stretch, the better the results.
Step 2: The pen is mightier
With the band stretch to its limit, place your message carefully on the band. It helps to place the letters close together. Neatness doesn't count.
Step 3: Time to relax
With the message complete, we can take the rubber band off the roll, and let it relax. The message will be unreadable to all but the keenest of eyes.
Step 4: Reading is fundamental!
To read the message, one need only stretch the band, and voila!