Take an irregularly cut cardboard and make three holes near the edges, at three different corners. Fix a nail on wall and make the cardboard to hang on it wi th the nail passing through one of the holes. Now tie a stone at one end of a piece of string and then allow it to hang from the nail. Draw a line on the cardboard in path of the string.
Then repeat the procedure making the cardboard to hang, through the other two holes and draw the lines each time. After the third time, you will find the lines intersecting at a particular point. This is the centre of gravity of the cardboard.
The Principle of Roof Building
Place two card boxes as shown. Take a hard cover book and place it on the boxes. You will find the boxes being pushed apart and then falling down. Now take two flat scales and place them on the boxes. Arrange the book again on them. You will find that the boxes are not falling apart. This is how the roofs and ceilings are built.