Modern-day dogs have retained some instincts of their wild ancestors. For Example, they turn around several times before lying in the grass. Their ancestors trampled tall grass to make a bed. Dogs also pull their tails between their legs when they are frightened. Their ancestors did the same to protect their tails from enemies.
Many rivers flow out to the sea.
They take the swiftest course. They flow the most quickly down the sides of mountains. They flow more slowly on flat plains. Rapidly moving water, particularly on a slanted surface, wears away land. This wearing-away process is called erosion.