What happens when you heat a blank piece of paper that has lemon juice on it? The juice dries clear and invisible. But when you apply heat, the writing turns brown and appears. You write a message on paper using lemon juice as ink. After it dries, you hold it near a hair dryer or another heat source. The heat causes the lemon juice to oxidize (react with oxygen) and darken. Your hidden message shows up as brown text on white paper.