Lemon juice and secret messages
Lemon juice contains sugars (carbohydrates) and acid (lemon acid). Sugars turn into carbon+water when heated in presence of acid. This is a standard thin layer chromatography analysis for sugars (spray plate with acid and heat up - black spots are sugars). The reaction is 

C12H22O11 (sucrose) + H$^+$ (catalytic) -> 12C (charcoal) + 11 H2O (steam).

Paper also has polysugars (cellulose), but cellulose is more robust. Eventually even paper will turn black by the same reaction.

You can use sugar + vinegar as the ink. Result will be the same. It proves that decomposition of sugars in presence of acid is the reaction responsible for revealing this ink.