A chord is a straight line joining two points on a circle. One particular chord passes through the centre of the circle, and it turns out to be longer than every other chord that circle has. What is that chord called?

Explanation

Every chord joins two points on the curve, but only the one running through the centre stretches the full width, and that chord is the diameter, which measures twice the radius. A line reaching from the centre to the curve travels only half that far, so it cannot be the longest chord.