Why do plants give out CO2 at nighttime?

During daylight, plants are undergoing a process known as photosynthesis. In that the plants take up carbon dioxide and in the presence of light use it to manufacture sugars. From these sugars they can manufacture all other organic molecules. A by-product, actually more a waste product, of this reaction is oxygen, which is released by plants.

Now, consider this. All living things need to respire and the general equation for respiration says that glucose and oxygen give carbon dioxide and water. Plants are also living and do respire. Only at day time, they have sunlight and drive their energies into making food for themselves from carbon dioxide. At night, there is no sun. So, photosynthesis cannot work. However, the plants do have to respire. So, they do just that and we see that at night they are taking in oxygen and giving off carbon dioxide just like us!

plants give out carbon dioxide all the time, not just at night. It's the conversion of nutrients into plant cells which has carbon dioxide as a result. During the day, with photosynthesis, the net gaseous outflow is in favour of oxygen, because the plant is taking in the carbon dioxide as part of its nutrients (it does so at night as well, but without the net production of more oxygen)