Squeeze lemon juice into a small bowl. Use the swab to write a secret message on the paper. Let it dry. To reveal, gently heat the paper by holding it near a light bulb or carefully over a candle flame (adult supervision required).

Place the bottle on the tray. Add 2 tablespoons of baking soda, a squirt of dish soap (for extra fizz), and a few drops of food coloring. Pour in about 1/4 cup of vinegar and watch the eruption!