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Teaching Guide
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Tickling in the Lab
Investigating the Yawn
Frozen Droplets
image of faucet

In "Grains of Inspiration," physicists Heinrich Jaeger and Sid Nagel help Alan Alda answer a host of little questions: Why does wet sand create a halo effect around your feet when you walk on the shore? Why do Brazil nuts always rise to the top of a can of mixed nuts? Why do coffee spills leave rings when they dry? But these are only a few of the many topics that interest these two scientists.

Another little question Jaeger and Nagel have been trying to answer: what is the actual shape of a water drop? Here's your chance to conduct your own research to find out. Often, things happen too quickly for us to accurately interpret. For example, the continuous motion of a movie or television show is actually a stack of separate images. Flipped at a fast enough rate, we get tricked into seeing smooth motion that really isn't there. The same is true for drops of water that drip from faucets. Most likely, you envision these drops as they elongate from the metal faucet rim. As they fall, they appear to retain a "teardrop" appearance before they slams into the basin. It's a nice visualization - too bad it's incorrect. (For more details on Jaeger and Nagel's findings, see the Web Connection below).

A stroboscopic disk is a device that can create the illusion of frozen motion. It does this by offering a quick "gated" look at a moving scene. If the gate is timed correctly, you see the same part of the event (but with a new subject) again and again. Through this limited and repetitive observation, you can perceive an illusionary halt to the ongoing motion. In this activity, you'll construct a stroboscopic disk and use it to "freeze" droplets that drip from a faucet.

note to educators



These activities will offer:

  • a hands-on experience in constructing a stroboscopic viewer
  • an opportunity to view motion that appears stopped due to stroboscopic effects
  • an understanding of how a gated view may produce the illusion of "frozen" motion


  • Heavy stock paper
  • Pencil with an eraser
  • Push pin
  • Flashlight (battery powered)
  • Access to a sink and cold water faucet.


  1. Trace or copy the template shown here onto a sheet of heavy stock paper.CLICK HERE FOR TEMPLATE
  2. Use scissors to carefully cut out the outline of this disk.
  3. Carefully cut out each of the slots that are positioned along the rim of the disk.
  4. Insert a pushpin into the center of the disk. Anchor the pin into the eraser of a pencil. (SE stroboscopic disc
  5. Twirl the disk to insure that it spins freely. If it binds up, move the pin around so that the hole widens.

Template for the strobe disk



  1. Work in a group of two students. Make sure that there are no AC-electrical devices near the sink.
  2. Identify the cold water faucet. Open and adjust the faucet so that a steady stream of large droplets falls into the sink.
  3. At this point, you may wish to dim the room. Aim the beam of the flashlight at the droplets. At the best illumination angle, the drops will be illuminated and separated from the sink basin.
  4. While your partner adjusts the flashlight beam, practice spinning the slotted disk with a constant and continual motion. Once a uniform speed is maintained, closes one eye and peers through the rotating slotted rim. As each slot rotates into view, it opens a quick "window" through which to see.
  5. Once the technique is mastered, the water droplets are viewed through the spinning slots. In order to FREEZE the droplet stream, you must spin the disk at a speed that is in sync with the falling droplets. At the correct speed, the slotted view will show the next drop in the same region of space as the previous drop.
  6. If the drops don't appear stopped in midair, don't give up. In order to perceive this illusion, the timing between the falling dorps and the rotating disk must be perfect. If this gating is not in sync, then your observation won't generate the illusion. Try changing the rate of the drip and/or the rotational speed of the disk until you observe the frozen droplets.
  7. Exchange roles.

image of faucet, girl and disc


  1. Why did you illuminate the drops in the flashlight beam?
  2. Why was the rotation speed critical to freezing the droplet stream?
  3. Suppose you rotated the disk too quickly. What would you see?
  4. Suppose you rotated the disk too slowly. What would you see?
  5. What shape did the "frozen" water drops appear to be?



Stop The Screen

The stroboscopic disk you created can by used to examine all sorts of phenomena. Did you realize that a monitor screen continually "redraws" its image? Obviously this happens at a rate too fast for us to detect. However, if you view the screen through the rotating slots of your view, you'll be able to view this refreshing process. At the right speed, you'll isolate and observe dark bands that extend across the monitor.

On Your Own

Find out how a strobe is used to "time" an automobile engine. If available, bring one of these engine strobe guns to class and discuss its operation.


University of Chicago
Read more about Jaeger's and Nagel's research at the University of Chicago

Strobe Photo Information
A biography of strobe photography pioneer Harold Doc Edgarton with strobe photo references

Scientists Discuss Water Droplets
Communication between scientists on the nature of water droplet size


The activities in this guide were contributed by Michael DiSpezio, a Massachusetts-based science writer and author of "Critical Thinking Puzzles" and "Awesome Experiments in Light & Sound" (Sterling Publishing Co., NY).

Academic Advisors for this guide
Corrine Lowen, Science Department, Wayland Public Schools, Wayland, MA
Suzanne Panico, Science Department, Fenway High School, Boston, MA

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A Ticklish QuestionLaughing MattersCold ComfortWhy are Peppers Hot?Grains of Inspiration Teaching guide Science hotline video trailer Resources The Sight of Touch Grow your own brain True or False What's in a dream Monastery of the Mind The Power of Half Contact Search Homepage video trailer Science hotline Teaching guide Resources Profile: Robert Edelman The Knowledge Michelle Geller The brain game