Definition: In an amusement park, I'm about to ride my roller coaster at a moment's notice. Suddenly, something unexpected happens: the coaster starts to roll backwards. I turn around and find myself flying through the air! What am I doing? My friends are laughing as I try to catch up with them. "We're going in circles!" They exclaim. "That's not how roller coasters work!" And so we laugh, until we finally manage to slow down and stop at a safe spot before dismounting, only to see our roller coaster completely overturned! Now that was an experience!