Howstuffworks – What is causing Dell laptop batteries to explode?>

Dell and the United States Consumer Product Safety Commission (CPSC) recently announced that 4.1 million laptop batteries could short-circuit under the right conditions, causing an explosion or a fire. The massive recall isn’t the first to hit the computer industry – HP and Apple both recalled laptop batteries between 2000 and 2006. But the threat of explosion does set it apart. How could something like a battery actually catch on fire?