The first three times are chosen so that you can explore your reaction time. Have another person hold a ruler vertically by one end. Place your finger and thumb at the lower end of the ruler, but don’t let them touch the ruler. Have the other person drop the ruler and you use your finger and thumb to grab it. Note the distance the ruler has dropped in the time it takes you to react to it being dropped. Compare the distance with the first three distances to see if your reaction time is between 0.1 and 0.2 seconds.