Tie a string to two stakes and pull chalk around them: every point of the oval has the same sum of distances to the two stakes, the string length. Putting the stakes at (±c, 0), calling the sum 2a and setting b² = a² - c² turns that sentence into x²/a² + y²/b² = 1. A constant difference of distances gives a hyperbola, x²/a² - y²/b² = 1 with c² = a² + b² and asymptotes y = ±(b/a)x. Circles and parabolas are the other two conics, reviewed by completing the square.