After a couple of hours sunbathing Sally decided to go and have a swim in the warm sea. She waited for David to come back. Unfortunately he did not so she just went into the sea. She had to walk out several hundred feet just for the water to go to her belly button. Then she did the back stroke. She swam in straight line across the beach. Never towards or away from it. After a few minutes she stops swimming and stands up. But she notices something weird.