Keeping water cool?

In Mediterranean countries, one can find ceramic containers that look unfinished to me. Where's the glaze covering? Won't it leak without some kind of outer seal over the ceramic?

The best known example is the Spanish 'botijo' and it is unglazed and leaky by design. Water slowly seeps through the ceramic and evaporates on the outside and this evaporation cools the contents within; up to 30 degrees F cooler in optimal conditions. We humans use the same technique to keep cool when sweat evaporates on our skin.

The botijo "jar" usually has a wide, spherical belly with two openings on top, a wide opening for pouring water in and a smaller opening as a drinking spout. The jar may also have one or two handles to carry it and tilt it. Traditionally people drink from a botijo by lifting it and tilting the spout to pour water into their mouth (but not touch the lips, that's considered very bad manners).

The evaporating-cooling trick can be applied in other clever ways. Most Army/Boy Scout canteens have a cloth outer cover or carrying bag. That cloth should be soaked when you fill the canteen and it will cool the contents as it dries. For water bottles you can make your own instant covering by placing the water bottle inside a thick, wet sock. If you try this I'd keep the bottle/sock strapped outside your backpack, not inside it.