ABOUT KIDNEY STONES: Kidney stones are made in the kidneys from materials that have crystallized out of urine, usually calcium and other chemicals that are part of the diet. They are very hard, and if they are small, they will often pass out of the body with urine unnoticed. When they are large, they can become caught in the tubes leading from the kidney to the urinary tract, which causes great pain. Most stones eventually pass without surgery, despite the pain. Doctors do not know what precisely causes kidney stones, though they do know that people who have had kidney problems or have a family history of kidney stones are more likely to develop them.
ABOUT GLOBAL WARMING: Global warming refers to an average increase in the earth's temperature, which has risen about 1 degree Fahrenheit over the past 100 years, and to changes in climate. A warmer earth may lead to changes in rainfall patterns, and a rise in sea level, for example, as the polar glaciers melt. Some of this rise is due to the greenhouse effect: certain gases in the atmosphere trap energy from the sun so that heat can't escape back into space. Without the greenhouse effect, the earth would be too cold for humans to survive, but if it becomes too strong, the earth could become much warmer, causing problems for humans, plants and animals.