1. Cover with mud: Cover the words with mud, and then remove the mud to reveal the words written in squid ink. This is a common cheating method in ancient times.

2. Bake with fire: After the words written in ink are dried, bake with fire to make the words written in ink appear.

It should be noted that the main component of squid ink is protein. If exposed to the air for a long time, the characters will decompose and disappear. So it needs to appear in a short time.