Create a secret Valentine using a plastic grocery bag and a permanent marker.
Begin by investigating what happens to the letters on the bag when the bag is stretched widthwise (across). Do the letters look misshapen? Then try stretching the bag in the other direction lengthwise. Do the letters appear normal again? You can make a secret Valentine message using this same idea. Cut a strip of the bag and write a message on the strip (widthwise) in permanent marker. Let the marker dry and then stretch the bag until the letters look misshapen being careful not to break the bag. Then put your secret Valentine message in an envelope with instructions about how to stretch the valentine back into shape and read the message.
Idea taken from Science Projects for Holidays Throughout the Year by Mickey Sarquis and Linda Woodward from Terrific Science Press Middletown, Ohio