Why do orangutans walk without sound? Brain teaser

The answer to this brainteaser is because orangutans walk quietly.

The orangutan walks without making any sound because the orangutan knocks its mimi. Gorillas are vegetarian and herbivorous, mainly eating fruits, leaves and branches. Found in the lowland and mountain rainforests of the Democratic Republic of the Congo, southwestern Uganda and Rwanda, as well as submountain forests, within the Lualaba River, Lake Edward and Lake Tanganyika. Lives in the jungles near the equator of the African continent.