Earthenware pots are the correct answer because they have properties that help keep the water cool. Let's explore why this is the case.

Properties of Earthenware Pots:
Earthenware pots are made from clay and have certain properties that make them ideal for keeping water cool. Some of these properties include:

1. Porous Material: Clay is a porous material, meaning it has tiny pores or small holes. These pores allow for the evaporation of water from the surface of the pot, which helps to cool the remaining water inside.

2. Natural Insulation: Clay is also a good insulator, meaning it helps to maintain a consistent temperature. The thick walls of earthenware pots provide insulation, preventing heat from the outside from entering the pot and warming the water.

3. No Heat Conduction: Unlike materials such as metal or glass, clay does not conduct heat easily. This means that even if the outside of the pot is exposed to sunlight or warm air, the heat does not transfer to the water inside as quickly.

How Earthenware Pots Keep Water Cool:
Now that we understand the properties of earthenware pots, let's see how they keep water cool:

1. Evaporative Cooling: As the water inside the earthenware pot evaporates through the pores in the clay, it absorbs heat energy from the remaining water. This process of evaporation requires energy, which is taken from the water itself, resulting in a cooling effect.

2. Insulation: The thick walls of the earthenware pot provide insulation, preventing heat from the surroundings from reaching the water inside. This helps to maintain a lower temperature for a longer period.

3. Shade: Earthenware pots are often placed in shaded areas, away from direct sunlight. This further reduces the heat absorbed by the pot and helps to keep the water cool.

Conclusion:
In summary, earthenware pots are ideal for keeping water cool because of their porous nature, insulation properties, and the process of evaporative cooling. The combination of these factors allows the water to remain cool for longer periods, making them a reliable source of cold water for thirsty travelers.