Most people are aware that an ordinary sewing needle can be made to float on water The experiment works best if the needle is slightly greasy. Usually, the needle is placed on a small piece of filter paper, which is then gently placed on the water surface.
Within a few seconds the paper sinks to the bottom and the needle is left floating Close examination reveals that the needle rests in a slight depression (Fig. 13.- The surface of the water behaves as though covered with an elastic skin. The property of a liquid is called surface tension. A piece of gauze cut and bent into form of a rectangular box will also float, but if a few drops of alcohol, soap solution or detergent are added to the water the surface tension is decreased and the and needle will sink. A tent keeps out water owing to the tension which exists in the lower surface the rain-water in contact with the canvas. Campers soon learn that it is important not to touch the inside of the canvas, or the surface film may be broken. When occurs, the water spreads round the strands of the material and, having nothing support it, begins to drip through.