Question

why the things written with lime juice become visible only by heating

Answer 1

Using the lemon as ink is the age old proposes to hide the messages within and send it to the receiver safely.

When the messages written in lime ink heated then the images gets visible to the eyes.

However, why this happen? When heating the paper then the ink or the limejuice wickens the paper fibres and thus it gets discoloured and in this way the messages gets visible to the eyes before the paper burn to ash.