Explain the following observations very briefly.
(a) A firki does not rotate in a closed area.
(b) The arrow of weather cock points towards a particular direction at a particular moment.
(c) An empty glass in fact is not empty.
(d) Breathing through mouth may harm you.

(a) A firki or pinwheel rotates when placed in an open area due to the moving air present all around. In a closed area, there is a lack of air movement which does not support the rotation of a firki.

(b) A weather cock is an instrument used for indicating the direction of the wind. It consists of an arrow, mounted at its centre of gravity so it can move freely about a vertical axis. Therefore, the arrow points towards a particular direction at a particular moment to show the latest direction of the wind movement.

(c) An empty glass is filled with the air. It is filled completely with air even if turned upside down. It can be proved with a simple experiment in which an empty bottle is turned upside down in a beaker filled with water. The water does not enter the bottle when it is pushed in an inverted position because there is no space for air to escape. When the bottle is tilted, the air is able to escape out in the form of bubbles and water fills up the empty space. This shows that even an empty glass is filled with air.

(d) Air contains some gases, water vapour and dust particles. When air is inhaled through nostrils, the fine hair and mucus present in the nose prevent dust particles from entering the respiratory tract. But, when we breathe through the mouth, the harmful dust particles may enter our body and make us ill. Thus, breathing through the mouth may harm us.