Ninja (or shinobi) were a mystery in the History of Japan. The correct Japanese word for these warriors was shinobi-no-mono, which literally means "people who survive/endure". Ninja is easier to say; this is why it is more widely used. Shinobi-no-mono is the native Japanese word for ninja, while ninja is the Sino-Japanese word. Ninja warriors created secret associations and took part in many political killings. Their mystery comes from two ideas: 1) they always participated in secret spy operations and political killings; 2) ninja were hired by heads of armies as paid warriors (mercenaries). The art of fighting which was used by ninja was called ninjitsu, which was a combination of shinobi-no-jitsu and shinobi-jitsu.