Running low: world leaders meet to discuss the planet's water problems.(Cover story)

It isn't easy for Mexico City resident Carmen Martinez to get water. She lives in Iztapalapa, one of the poorest slums of the Mexican capital. A slum is a crowded, dirty area with poor living conditions within a large city.

Once every eight days, she wakes up before dawn and treks down a hill from her run-down shack to a pumping station to beg for water. She stands in line with as many as 150 other people. When her turn finally comes, she gets aboard a tanker truck full of brownish, smelly liquid.

The truck then climbs the hill and pours the water into a dumpster and six barrels. Martinez and her family use that water to bathe, clean, and wash. Although the water is free, it is not potable, or drinkable, so Martinez also buys bottled water at a store. She imagines what it would be like "to just turn on a faucet."

Representatives from 148 countries recently met in …