The answer “Because it contains the substance chlorophyll, which is green” is not a lie and gives the child the necessary answer. But it does not have the necessary knowledge. The fact that chlorophyll absorbs everything except green is the definition of green. From this point of view, the green cube is green only because it reflects the green color. And not because it was painted with green paint.
And now, let’s do it like a child. Sit your child on a chair, tie it and begin to explain.
So, the grass is green, because it contains chlorophyll. Chlorophyll is needed to provide the process of photosynthesis and convert carbon dioxide to oxygen, while obtaining energy for the life of the plant. And it is with the process of photosynthesis that the green color of chlorophyll is associated.
White light is a mixture of all colors of the visible part of the spectrum. Each color has its own wavelength of light. Energy is inversely proportional to the wavelength: the longer the length, the less energy. This can explain why chlorophyll absorbs blue colors, because they have the greatest energy.
The absorption of red colors depends on another characteristic of light radiation – the number of photons per unit of energy. Chlorophylls are activated not by energy, but by photons. That is, the more photons are more, the more active the photosynthetic reaction takes place. The number of photons, on the contrary, increases with increasing wavelength.
As a result, chlorophyll is much more beneficial to absorb red and blue colors than colors in the middle of the spectrum, which give the plant too little energy. Therefore, chlorophylls reflect it. And we perceive the reflected color as green.
Compared to real physics, this is a rather simplistic approximation, but still not sufficient for the child.
You can say this:
“The grass is green, because with this color it can receive more heat from the sun and it is better to grow.”