The most convincing explanation is that when someone is finding it difficult to understand what is said to them, he/she engages in a form of lip-reading: he/she will observe the lips of the talker and mimic the talker's lip movements.
Lake Tanganyika is an African Great Lake. It is estimated to be the second largest freshwater lake in the world by volume, and the second deepest, in both cases, after only Lake Baikal in Siberia; it is also the world's longest freshwater lake.
The largest lake in the world is the Caspian Sea. It has a surface area of 143,200 square miles. It is also the largest lake in the world by volume, containing 18,800 cubic miles of water. This oceanic lake is bordered by Russia, Iran, Kazakhstan, Turkmenistan and Azerbaijan.