How do you make invisibe ink appear?

To make invisible ink appear, you typically apply heat or a chemical reagent. For example, if the ink is made from lemon juice, you can hold the paper over a heat source, such as a light bulb or a candle flame, causing the ink to oxidize and turn brown. Alternatively, using a chemical like baking soda mixed with water can reveal the ink when painted over with a certain indicator, such as Grape Juice. Always exercise caution when using heat or chemicals.