What is invisible ink and why?

Dilute sulfuric acid is called invisible ink. The tip of a stick is dipped in dilute H2SO4 and something is written on the paper. After some time the writings are no longer visible when the paper dries. Now if you hold the paper in the heat of the fire, the writings of black letters will emerge. Hot sulfuric acid absorbs the water content from the paper. As a result, only carbon remains.