Here is a way to use baking soda to make invisible ink. Mix equal parts of water and baking soda. Use any instrument that you choose to transfer your baking soda ink message to a plain piece of paper. Allow the paper to dry. When you want to read your secret message hold the paper to a heat source such as a light bulb. The heat will cause the baking soda ink to turn brown. Another method of reading your message would be to paint your paper with grape juice. The message will turn to a different color.