While it is true that water in an uncovered container will eventually evaporate, it does so much more slowly than you might think. The rate of evaporation depends on a number of factors, including the type of container, the temperature of the water and the humidity of the air. In a glass or plastic container, for example, evaporation can occur quite quickly if the water is hot and the air is dry. In a metal container, however, evaporation happens much more slowly because metals are good conductors of heat and they tend to keep their contents at a consistent temperature.