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.
1 The largest lake in the world by area is the Caspian Sea, with 394,299 kmÂ². The largest freshwater lake by area is Lake Superior (82,414 kmÂ²), part of the Great Lakes of North America.  The longest freshwater lake is Lake Tanganyika, with a length of about 660 km.