In your childhood, you must have sat on a Rope swing a wooden cradle tied with coir ropes to the sturdy branch of an old tree. The more modern versions of these swings can be seen today in the children's park in urban areas, they have a plastic cradle tied with steel chains to steel frameworks. Consider a rope swing that is tied identically with two equal ropes. It swings equally, when you sit in the middle of the cradle. Buildings too are like these rope swings; just that they are inverted swings . The vertical walls and columns are like the ropes, and the floor is like the cradle. Buildings vibrate back and forth during earthquakes. Building with more than one storey is like rope swings with more than one cradle.