What was the cause of the space shuttle blowing up in 1986?

The space shuttle Challenger was destroyed when one of the the large white Solid Rocket Boosters (aka SRB) failed. A failure in one of the SRB's joints caused a leak. That leak burned a hole in the orange External Tank (aka ET.) The leak also burned the connection of the SRB to the ET. Once that connection failed the SRB separated from the ET and pierced the upper portion of the ET. When that happened the fuel in the upper tank mixed with the fuel in the lower tank and ignited. The fireball and explosion blew the Challenger into pieces.