Why did the space shuttles explode?

The breakup of the space shuttle Challenger was caused by the failure of two O- rings in one of the solid rocket boosters (SRB's) to properly seal. There had been problems with the seal on other missions, but it is thought that the decision to launch in near-freezing temperatures contributed to the failure, making the seal rigid and unable to seal properly. This led to a catastrophic chain of events. Hot gases escaped from the SRB, followed by a flame, damaging the clamp securing the SRB, and burning through the external fuel tank causing the tank to disintegrate. The forces created caused the orbiter to disintegrate (it did not explode) before the debris crashed into the ocean. The Columbia Space Shuttle disintegrated upon re-entry into the Earth's atmosphere on February 1, 2003. The Columbia sustained damage to its thermal protection system during launch. NASA suspected damage to the orbiter, but did not believe it was enough to cause any danger. The damage allowed hot gases to penetrate and destroy the internal wing structure, causing the shuttle to break up upon re-entry.