Question

Why are a plants stomata generally open during the day and closed at night?

Answer 1

It's open because of the sunlight, have you heard of Photosynthesis?
That's what it is doing, if you notice plants grow towards the sun, because they feed themselves.
So the answer is,
It undergo's photosynthesis, so it is open during the day for the sun to produce food.

Answer 2

Wouldn't they open for Photosynthesis? :\ I mean at night they don't need them open. There is no sunlight