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 photo above shows a view of Lake Titicaca, which at 12,500 feet (3,811 m) above sea level is the highest commercially, navigable lake in the world. In terms of volume, it's also the largest lake in South America. Lake Titicaca is located on the border between Peru and Bolivia.