Ninjas were spies,they had to remember all the detailed information they acquired on a mission sometimes for weeks or months. So they developed a few techniques to easily remember things
- Numbers associated with body parts: Each body part represent a number from 0 to 9 (1 eyes, 2 nose , 3 mouth, 4 throat, 5 elbow, 6 hair, …). This made it easier for ninja to remember any particular number (e.g. 431=throat-mouth-eyes)
- Scars on the body: This sounds painful but the ninja used to lightly carve the important info on their skin by using sharp objects like shuriken.
- Hidden paper notes in bamboo sandals: To look normal the ninja often wore sandals made out of bamboo leaves. After writing the notes on tiny pieces of paper they used to roll it tighty and attach it to their sandal as if its a bamboo straw.