How to Make an Invisible Ink Message
To make an invisible ink message that will be revealed by heat, first pour 1 teaspoon (5 mL) of lemon juice into a small bowl. Add 1/2 teaspoon (2.5 mL) of water and mix it with a spoon. Take a sheet of white paper and place it on top of a towel. To write your message, take a cotton swab and dip it into the lemon juice and water. Write with the tip of your cotton swab, dipping the cotton swab into the lemon juice after every 2-3 letters to give it fresh ink. Once you've written your message, wait 2-3 minutes to give your paper time to dry. For the message to be revealed, hold the paper up to a hot lightbulb or leave it on a radiator to heat the paper. As the heat burns off the lemon juice, the message will be revealed!
Squeeze the juice of half of a lemon into a bowl.  Add a few drops of water. Stir the juice and water with a spoon. Dip a cotton swab into the lemon juice mixture and write a message on a piece of white paper. A feather, a toothpick, a fountain pen , a paintbrush or a calligraphy pen will also work in place of the cotton swab. Allow the invisible ink to dry. As the ink dries, your message will disappear. Hold the paper that contains your invisible ink message over a light bulb or flame/candle. Keep the paper near the bulb or flame/candle until the message reappears. Be careful not to put the paper too close if you are using a flame/candle. Dip a cotton swab into a mixture of 1/4 cup (60 ml) of baking soda and 1/4 cup (60 ml) of water. Use the swab to write a message on a white piece of paper. Wait until the ink is dry.  Dip a paintbrush into some grape juice concentrate or any dark juice. Paint the grape juice over your message and wait for the message to appear. Dip a cotton swab into a shallow dish of milk.  Use the cotton swab to write a message onto a white piece of paper. Allow the milk to dry. Dry the message for about 30 minutes.  Expose the message to heat. You can use a light bulb, candle/flame, or an iron. The milk will heat up at a slower rate than the paper, which will cause the message to reappear. Use a white crayon to write a message onto a white piece of paper.  Paint over the message using watercolors and a brush. The message will reappear. Lay out two sheets of paper. Place the first sheet of paper under the second piece. Write your message on the top paper. Do this by applying strong pressure on the pen or pencil. Remove the first sheet.  With a pencil, gently shade over the bottom sheet. The message will appear, similar to chalk on a blackboard. 
Squeeze the juice of half of a lemon into a bowl. Add a few drops of water. Dip a cotton swab into the lemon juice mixture and write a message on a piece of white paper. Allow the invisible ink to dry. Hold the paper that contains your invisible ink message over a light bulb or flame/candle. Dip a cotton swab into a mixture of 1/4 cup (60 ml) of baking soda and 1/4 cup (60 ml) of water. Wait until the ink is dry. Dip a paintbrush into some grape juice concentrate or any dark juice. Dip a cotton swab into a shallow dish of milk. Use the cotton swab to write a message onto a white piece of paper. Dry the message for about 30 minutes. Expose the message to heat. Use a white crayon to write a message onto a white piece of paper. Paint over the message using watercolors and a brush. Lay out two sheets of paper. Write your message on the top paper. Remove the first sheet. With a pencil, gently shade over the bottom sheet. 