Key concepts:
Reaction time

Think fast! Have you ever noticed that when someone unexpectedly tosses a softball at you, you need a little time before you can move to catch it (or duck)? That's because when your eyes see an incoming signal such as a softball, your brain needs to first process what's happening—and then you can take action. In this activity, you can measure just how long it takes for you to react, and compare reaction times with your friends and family.

You may not realize it, but when your senses pick up clues from the outside world—the smell of baking cookies, the color of a stoplight, the rrrring! of an alarm clock—it takes a fraction of a second for you to recognize that signal and respond. During that time your brain receives information from your senses, identifies a possible source, and allows you to take action. The jam-packed fraction of a second is called your reaction time.

This activity teaches you about your brain's reaction time, but it also relies on the laws of physics. Specifically, you can calculate your reaction time using our handy chart, which is based on how quickly a ruler falls. How do we know how quickly your ruler will fall? Gravity pulls all objects toward Earth's center at the same speed. If you want to try this out at home, try dropping a tennis ball and a basketball from the same height: They should both hit the ground at the same time!

·    Ruler (inches or metric)
·    Paper
·    Pencil
·    Chart (below)

Line Where Partner Pinched Ruler
(inches | centimeters)
Reaction Time
(seconds | milliseconds)
2 in. 5 cm. 0.1 sec. 100 ms.
4 in. 10 cm. 0.14 sec. 140 ms.
6 in. 15 cm. 0.17 sec. 170 ms.
8 in. 20 cm. 0.2 sec. 200 ms.
10 in. 25.5 cm. 0.23 sec. 230 ms.
12 in. 30.5 cm. 0.25 sec. 230 ms.
17 in. 43 cm. 0.3 sec. 300 ms.
24 in. 61 cm. 0.35 sec. 350 ms.
31 in. 79 cm. 0.4 sec. 400 ms.
39 in. 99 cm. 0.45 sec. 450 ms.
48 in. 123 cm. 0.5 sec. 500 ms.
69 in. 175 cm. 0.6 sec. 600 ms.

•     You need to use some math skills in this challenge. To make things easier, we've provided a chart, above, that you can print or copy out on a piece of paper. The basic rule: 100 milliseconds translates into about two inches or five centimeters.
•     On a clean sheet of paper, write the name of each person—including yourself—who will take part in this experiment. You only need two people for this activity, but it's also great for a group. Leave five spaces below each name.

•     Hold the ruler vertically so that the zero end hangs down.
•     Ask your partner to stand next to you and place his or her hand below the ruler's zero line, ready to catch the ruler when it falls by pinching it between his or her thumb and index finger. Your partner's fingers should be just below the ruler, but as close as possible to the bottom edge without touching or overlapping.
•     Tell your partner that you will count from one to five and drop the ruler at some point during the count. Your partner will need to catch the ruler as quickly as he or she can, pinching the ruler between his or her fingers.
•     Count from one to five and drop the ruler at some point
•     Your partner should catch and pinch the ruler. How fast did your partner appear to act? Did your partner's fingers pinch near the zero line?
•     Write down the centimeter or inch line where your partner's fingers pinched the ruler.
•     Calculate how long it took your partner to respond using the chart provided. Was your partner as fast as you thought?
•     Repeat the drop four more times for your partner, and record the measurement each time. Does your partner's reaction time change? Are the five reaction times different? Vary when you drop the ruler: For example, you could drop on the count of five first, then drop on two.
•     Switch tasks and try catching when your partner drops the ruler, then compare your results with the others. Do most people have a similar reaction time? Are older people faster than younger people? Are girls faster than boys?
•     You can also try a few variations: What happens when you tell your partner when you will drop the ruler? Does reaction time improve with practice?
•     Extra: Ambidextrous, anyone? Repeat this activity and compare your results when you use your dominant hand—the hand you write with—and when you use your other hand. Is there any difference between hands?
•     Extra: Consider adding other distracting sounds and sights—such as turning on a TV set or flicking a flashlight on and off—during the activity. Do your responses slow with so many sensory signals?

Observations and results
Did you and your partner usually catch the ruler around 15 centimeters (six inches)? What took so long?

On average, reaction time takes between 150 and 300 milliseconds. If that sounds like a long time, think about how much has to happen for you to react. When your eye sees the ruler falling, information travels from sensory cells called neurons from the eye to the brain's visual cortex, an area devoted to understanding what you see. Next, the motor cortex—the part of the brain that directs movement—has to send signals along your spinal cord and to your arm, hand and finger muscles, telling them to respond in the proper sequence to catch the ruler—quick! That's a lot happening in less than half a second—and a pretty amazing feat!

More to explore:
Experience versus Speed from Scientific American MIND
Brain Brakes Car Faster Than Foot from Scientific American
Reaction Time Test from the Human Benchmark
How fast are your reactions? from the BBC