Illustration Photo: Boats used by tourists to explore Tonle Sap (Great lake in Cambodia). The boats are driven by a very long shaft with the propeller at the end as the water is very shallow in places. The Tonlé Sap is unusual for two reasons: 1) its flow changes direction twice a year, and 2) the portion that forms the lake expands and shrinks dramatically with the seasons. (credits: Dennis Jarvis / Flickr Creative Commons Attribution-ShareAlike 2.0 Generic (CC BY-SA 2.0))