Using Physics to Write Secret Messages

The earliest Valentine’s Day card still in existence is in the British Museum. The card was sent in 1415 by Charles, Duke of Orleans, to his wife while he was imprisoned in the Tower of London. Possibly there was a hidden meaning in the Duke’s message.

You can make a special valentine with a secret message. Use the evelope pattern shown here to make two valentine cards on a sheet of copy paper. 

Cut around the outside of each envelope, then cut out the heart-shaped piece. Fold the bottom section of the envelopes and the two tabs along the folded lines. Use small pieces of tape to secure the tabs as shown so you can have two small envelopes.
Turn the envelope so that the heart-shaped cutout is faceup. With a yellow felttip pen, print a secret message, such as the one shown, inside the heart. Cut out a square of plastic from a red, transparent, plastic report folder to fit in the envelope. Insert the red plastic square in the envelope. Note that your secret message disappears behind the red plastic. Tell the recipient of the valentine to remove the red plastic to read your secret message.

For a science activity about the heart see HEARTBEAT.

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