Most people have heard that you can't drink the water in Mexico, but most don't really know why. Most cities in Mexico have a good water delivery infrastructure much like the United States and other developed countries. However the water is only run for a few hours at a time. So your house may only have water coming in through the pipes from 8 to 10 pm.

Because of this you have to store the water in a cistern under the house or parking area. When water comes through the pipes the cistern fills up and then shuts off. Of course with the water underground, there is no way for it to have enough pressure to do anything useful.

To overcome this, every house has a small water tank on top. A pump moves the water from the cistern to the water tank whenever it gets low. In addition to providing water pressure, the tank offers additional water storage for use during the 22 hours a day that water is not flowing through the pipes.

Now do you see the problem? Even if the water comes in crystal clean, it is sitting in the underground storage tank for awhile and then sitting in the water tank on top of the house for a while. Either of these areas can be a place where bacteria breeds and in really bad cases, animals and bugs can get in an drown in the water.

Because of the way water is transported and stored it isn't safe to drink--even if the water coming in as perfectly safe. In some cases the incoming water isn't safe to drink either. Especially in Mexico City that is build on an old lake. The ground is very unstable so it is common to have small earthquakes. Sometimes the water lines and sewer lines run close together and if a shift in the earth causes them to break it can cause problems. Since the water lines aren't pressurized all the time, a break may let in sewage instead of just seeping water out into the ground. Obviously this can cause a problem for drinking water.

Most cities have a lot of bottle water for sale. You can get it from local shops or have it delivered to you. It generally costs about $1.50 US, for 5 gallons of water, so it isn't terribly expensive--especially since it is only needed for drinking and cooking.