When the Brontosaurus walked through lakes feeding on plants, your glass of water was part of those lakes. When kings and princesses, knights and squires took a drink from their wells, your glass of water was part of those wells. The earth has a limited amount of water. The water vapor or steam leaves the river, lake or ocean and goes into the air. Transpiration is the process by which plants lose water out of their leaves.
Transpiration gives evaporation a bit of a hand in getting the water vapor back up into the air.
Water vapor in the air gets cold and changes back into liquid, forming clouds. This is called condensation. Water forms on the outside of the glass. It actually came from the air. Water vapor in the warm air, turns back into liquid when it touches the cold glass.