You can compare your reaction time with that of a friend by catching a ruler that is dropped between your fingers. Let a friend hold the ruler as shown and you snap your fingers as soon as you see the ruler released. The number of centimeters that pass through your fingers depends on your reaction time. You can express the result in fractions of a second by rearranging d = ½ gt². Expressed for time, it is t = √2 d/g = 0.045√d, where d is in centimeters.