Nailed It

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Physics, Engineering: Center of Gravity, Problem Solving
Grade Range: Middle School, High School
Format: Hands-on

This puzzle will keep students engaged for a while as they try to figure out the secret to doing it. Be sure to bring several blocks for students to use, otherwise you won't have enough for them to share!



  • Small Wood Blocks
  • Large Nails (1 per block)
  • Small Nails (20 per block)

Safety Precautions

Please read the Physical Demonstration section of the Demonstration Safety page before performing this demonstration.


  1. Set out the parts, keeping the small nails in a bin or shallow dish. As students come up, ask them to try and stack 20 nails on top of the big nail.
  2. Allow students to try and figure out the puzzle. If they ask for a hint, let them know that they need to "Put some nails on a nail with a nail on top to get the nails on the nail."
  3. If any students figure out the trick, ask them to explain how they found the answer. Otherwise, if students start to give up, show them how to lay out the nails, and then ask them to try again to get the nails on top.

Why This Works

When the nails are being placed on top of the larger nail, students need to be careful about where the weight is distributed. The Center of Gravity for the nails is centered based on where the weight and any forces acting on the nails is distributed. By being careful, students will find that they can carefully get the nails to balance on top of the larger nail by having their center of gravity on the head of the larger nail.

As they try to solve this demonstration, students will find that they need to use Problem Solving skills to get to the answer. They first need to Define what the problem is. Students then go into Exploring options to solve the problem, and Applying the options that they think will work best for the problem. After they apply each option they come up with, they then need to Review the choice they went with, and use the results to find the correct method to solve the problem.

To solve this problem, students will have to follow the steps in a way similar to the following:

  • Define: Try to balance 20 small nails on the large nail.
  • Explore: The 20 nails look too wide to stack directly on top of the large nail. To get the 20 nails on top, they will have to be spread out and be able to hold each other up. This could be done by having several nails laid across a single nail, with the nail heads holding each other in place.
  • Apply: Lay out a single nail, and place nail heads on the nail, having the nail heads go between sides to lock each other in place. Once this is done, place the last nail on top, down the center. pick up all the nails by the ends of the two laying across, and carefully lift it and place it on the large nail's head.
  • Review: The method did work, and the 20 nails are all balanced on top of the nail head.

Additional Information

  • This demonstration pairs well with the Magic Spoon demonstration.
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