Scientific reasons
1.
If blood is red, why are veins blue?
Blood is bright red in its oxygenated form and a dark red in deoxygenated form. In simpler terms, it is bright red when it leaves the lungs full of oxygen and dark red when it returns to the lungs for a refill. Veins appear blue because light penetrating the skin is absorbed and reflected in high energy wavelengths back to the eye. Higher energy wavelengths are blue.
2.
Why is the sky blue ?
When sunlight travels through the atmosphere, it collides with gas molecules. These molecules scatter the light. The shorter the wavelength of light, the more it is scattered by the atmosphere. Because it has a shorter wavelength than the other colours, blue light is scattered more, ten times more than red light, for instance. That is why the sky is blue.
Why does the setting sun look reddish orange? When the sun is on the horizon, its light takes a longer path through the atmosphere to reach your eyes than when the sun is directly overhead. By the time the light of the setting sun reaches your eyes, most of the blue light has been scattered out. The light you finally see is reddish orange, the colour of white light minus blue.
3.
Why do onions make you cry?
Onions, like other plants, are made of cells. The cells are divided into two sections separated by a membrane. One side of the membrane contains an enzyme which helps chemical processes occur in your body. The other side of the membrane contains molecules that contain sulfur. When you cut an onion, the contents on each side of the membrane mix and cause a chemical reaction. This reaction produces molecules such as ethylsufine which make your eyes water.
To prevent crying when you cut an onion, cut it under a running tap of cold water. The sulfur compounds dissolve in water and are rinsed down the sink before they reach your eyes. You can also put the onion in the freezer for ten minutes before you cut it. Cold temperatures slow down the reaction between the enzyme and the sulfur compounds so fewer of the burning molecules will reach your eyes.
4.
Why does wet fabric appear darker?
When fabric gets wet, light coming towards it refracts within the water, dispersing the light. In addition, the surface of the water causes incoherent light scattering. The combination of these two effects causes less light to reflect to your eyes and makes the wet fabric appear darker.