Here's why:

* Photosynthesis requires sunlight: Photosynthesis is the process by which plants convert sunlight, water, and carbon dioxide into glucose (sugar) for energy. Without sunlight, this process cannot occur.

* Plants still need energy: Even in the dark, plants still need energy for processes like respiration, growth, and repair.

* Plants use stored glucose: During the day, plants produce more glucose than they need immediately. They store the excess glucose in the form of starch. When the sun goes down, they break down this stored starch to release energy for their essential functions.

In simpler terms:

Think of photosynthesis like a solar panel charging a battery. The sunlight is the energy source, and the battery is the stored glucose. During the day, the panel charges the battery. At night, the battery powers the plant's functions.

It's important to note: Some plants, like cacti, have adapted to survive in environments where sunlight is scarce. They use a different type of photosynthesis that allows them to store carbon dioxide during the night and use it for photosynthesis during the day.