Sir, why do plants need sunlight? I thought they make their own food, so why can't they just do it in the dark?

Plants need sunlight because photosynthesis, the process by which they make their food (glucose), is an energy-intensive reaction. Chlorophyll, the green pigment in leaves, traps light energy from the sun. This light energy is then converted into chemical energy, which is used to combine carbon dioxide from the air and water from the soil to synthesize glucose. The overall reaction is: $6CO_2 + 6H_2O + \text{Light Energy} \rightarrow C_6H_{12}O_6 + 6O_2$. So, while plants "make" their own food, they don't create energy; they convert light energy into chemical energy stored in glucose. Without sunlight, this crucial energy conversion cannot happen, and the plant cannot produce its own nourishment.