A hydrometer is an instrument used to measure the specific gravity of liquids. It is a weighted glass tube with a scale marked in specific gravity units. The hydrometer is placed in the liquid to be measured, and the point at which it floats is read off the scale.

Surface tension is the force that causes the surface of a liquid to behave like an elastic membrane. It is this force that allows rain-proof coats and tents to keep water out. When water hits the surface of a rain-proof coat or tent, the surface tension of the water causes it to bead up and roll off. This prevents the water from soaking through the material and getting the person wearing the coat or tent wet.

The sky appears blue because some of the blue component of sunlight is scattered by gas molecules in air. Sunlight is made up of all the colors of the rainbow, which are different wavelengths of light. Blue light has a shorter wavelength than other colors, so it is scattered more easily by the gas molecules in the air. This is why we see a blue sky most of the time.