I remember being slightly giddy the first time I was told that the equation of the line with x-intercept a and y-intercept b is $\frac xa + \frac yb = 1.$ I love this equation and the generalizations it implies. My guess is that less than one percent of the algebra students in a school would be impressed with this equation. So, occassionally, I teach to the one percent. Not really for them, but because I get a kick when that one student gets that look on his/her face.