25 years ago today, the space shuttle Challenger exploded shortly after launch, killing the 7 astronauts aboard. One of those astronauts was Christa McAuliffe, a high school social studies teacher. My own reminiscences aren’t all that important, so I’ll spare you, but please visit the NASA Day of Remembrance website for more information and commemorative addresses from President Obama and NASA Administrator Bolden. (Warning: the site has music that plays automatically.)
Galileo's Pendulum 