The water your teacher drank could have been from a glacier due to the water cycle, involving evaporation, condensation, precipitation, infiltration, and transpiration.
The teacher's claim that the water they drank could have once been part of a glacier in Greenland is entirely plausible due to the water cycle. This cycle details how water can move between different states and locations on Earth via various processes.
Frozen water from Greenland's glaciers can sublimate or melt and evaporate, turning into water vapor which rises into the atmosphere. It then cools and undergoes condensation to form clouds. The condensed water falls as precipitation (rain or snow), which may land in the Baltimore County Reservoir. Some precipitation infiltrates the soil, replenishing groundwater, which might be extracted for use, including drinking water in schools. Additionally, plants contribute to this cycle through transpiration, releasing water vapor back into the atmosphere.
This continuous movement of water is powered by solar energy, which drives the processes of evaporation and transpiration, and by gravity, which directs the flow of precipitation and groundwater.
Therefore, it is entirely feasible that a molecule of water enjoyed by the teacher in class had its origins thousands of years ago within the ice of a distant glacier. By understanding the water cycle, it is clear how water can traverse vast distances and times, changing form as it moves from glaciers to reservoirs to taps.