Cut three or four fresh lemons in half and squeeze the juice into a clean, dry bowl. Remove any lemon seeds that go in the bowl with a fork and discard.
Fold one piece of white cardboard stock paper in half to create a greeting card. You can purchase white cardboard stock paper at any office supply store or large chain book store.
Dip a pen brush or small paint brush into the lemon juice and write your greeting in the inside of the card. Draw illustrations in the card as well if you want, making sure to re-coat your pen brush. Let the magic greeting card dry for about 30 minutes.
Write the directions on how to read the card on the back of the envelope. Your recipient has to expose the card to a strong light source in order to read it. For example: "Expose the card to strong sunlight or hold it 2 to 3 inches away from a light bulb to see the writing."
Repeat the above steps to make more magic greeting cards. Place the magic greeting card in the envelope and send it to your friend or loved one and wait to hear her reaction after reading your card.