On a sunny morning, Dhanush and Mahi tie a clear plastic bag over a leafy branch of a potted plant

Why

Dhanush is correct that air carries moisture — that is where dew comes from. But a bag tied shut lets very little outside air in, and if outside moisture were the cause, a bag with no plant inside would fog up just as much. The real source is the plant. Water absorbed by the roots travels up through the stem to the leaves, and much of it escapes as water vapour through tiny pores on the leaves called stomata. This loss of water from leaves is transpiration. Inside the closed bag the vapour can't drift away, so it collects and condenses into droplets on the plastic. Transpiration also helps pull more water up from the roots. Test: at the same time, tie an identical bag over a dry, leafless stick pushed into the same pot — it stays almost dry, while the bag on the leafy branch fogs up. Mahi is right: the water came from the leaves, given off by transpiration.