There may be some moisture on the glass when the terrarium gets warm. That means the plants are transpiring more, and the water droplets on the glass are an example of condensation. A terrarium does not need very much water. You may not need to water your terrarium for several months if the soil is moist from the start. Up to 80 gallons of water per day can be transpired from an average sized tree.
Water enters the air either by evaporation or transpiration.
Precipitation percolates into the ground and becomes available to plants, or falls directly on rivers, streams, and lakes to begin the cycle again. Evaporation takes place because of the sun's heat. The water found in the can at the end of the experiment was formerly held in the soil. The dark plastic heats the soil during the day. At night, the temperature of the plastic drops faster than the soil temperature, causing moisture to condense on the plastic.