Cup Stack Claw

📅 July 29, 2024

Grade Level: 2-6
Time: 45-60 minutes
Group Size: 2-4 students per team

Materials Needed (per team):

• 10-15 popsicle sticks
• 2-3 yards of string or yarn
• 4-6 rubber bands
• Tape (masking or duct)
• Scissors
• Cardboard scraps (optional, for reinforcement)
• 3 plastic or paper cups (solo cups work great)

The Challenge:

Design and build a mechanical claw that can pick up and stack 3 cups without touching them with your hands. The claw must be controlled by pulling strings - no direct touching allowed once the challenge starts!

Step-by-Step Instructions:

Setup (5 minutes):

1. Set up a “building zone” for each team
2. Place 3 cups on the table spread apart
3. Explain the goal: stack all 3 cups into a tower using only the claw
4. Demonstrate: show students a simple grabbing motion (though they’ll build their own design)

Planning Phase (10 minutes):

1. Show students how a claw works:
   • It needs to OPEN to go around the cup
   • It needs to CLOSE to grip the cup
   • Strings control the opening and closing
2. Basic claw concepts:
   • The Gripper: Two or more “fingers” that come together
   • The Handle: Something to hold while operating
   • The Controls: Strings that open/close the fingers

Building Phase (30-35 minutes):

Design Option 1: Simple Pincher Claw (Easier - Grades 2-3)

What you’re building: Two popsicle sticks that work like chopsticks

1. Create the Fingers:
   • Take 2 popsicle sticks
   • These will be your grabbing “fingers”
2. Add the Hinge:
   • Overlap the sticks at one end (about 1 inch overlap)
   • Wrap a rubber band around the overlap tightly
   • The sticks should be able to open and close like scissors
3. Add Control Strings:
   • Tie a string to the end of each stick (the ends that open/close)
   • Make strings about 12-18 inches long
   • Pull strings apart = claw opens
   • Release strings = rubber band pulls claw closed
4. Add a Handle (optional):
   • Tape a third popsicle stick perpendicular to the hinge as a handle

Design Option 2: Multi-Finger Claw (Harder - Grades 4-6)

What you’re building: A claw with 3-4 fingers that close around the cup

1. Create the Base:
   • Use cardboard to cut a small circle (3-4 inches diameter)
   • This is the “palm” of your claw
2. Attach the Fingers:
   • Use 3-4 popsicle sticks as fingers
   • Attach one end of each stick to the edge of the cardboard circle
   • Space them evenly around the circle
   • Use tape or small hinges made from paper clips
3. Add the Closing Mechanism:
   • Thread ONE long string through small holes at the tip of each finger
   • OR tie individual strings to each finger
   • When you pull the string(s), all fingers should move inward toward center
4. Create the Handle:
   • Attach a long stick or rolled cardboard tube to the center of the base
   • This lets you hold the claw above the cups
5. Test the Grip:
   • Pull string = fingers close
   • Release = fingers open (you may need rubber bands to help them spring back)

Teacher Facilitation:

During Building (walk around and guide):

• “Show me how your claw will close. What makes it close?”
• “How will you control it from far away?”
• “Try picking up a cup right now - does it work?”

Common Issues:

• Claw won’t close tightly enough: Add rubber bands to pull fingers together
• Can’t grip the cup: Make sure fingers curve inward, not straight
• Cup slips out: Add texture to fingers (wrap with rubber band, add tape sticky-side-out)
• Strings tangle: Use different colors or keep them separated

Testing Phase (15 minutes):

The Cup Stack Challenge - Official Rules:

1. Place 3 cups spread out on the table
2. Using ONLY the claw (no hands touching cups!), stack all 3 cups into a tower
3. Tower must stand for 5 seconds
4. Fastest time wins! (Or award points for successful stacking)

Scoring Options:

• Completion: Did you successfully stack all 3 cups? (Yes = 10 points)
• Speed: How long did it take? (Fastest = bonus points)
• No Drops: Stack without dropping any cups (bonus points)
• Creativity: Most creative claw design (teacher’s choice award)

Learning Objectives:

• Simple machines: Levers and mechanical advantage
• Cause and effect: Pulling string causes fingers to close
• Engineering design: Iterating to improve grip and control
• Hand-eye coordination: Controlling the claw precisely

Differentiation:

• Easier:
  • Allow stacking just 2 cups
  • Make a simpler 2-stick pincher design
  • Let students touch cups to position them at the start
• Harder:
  • Stack cups upside-down (narrower opening!)
  • Build a claw that can pick up other objects (small ball, eraser)
  • Add a height requirement: claw must lift cup 6 inches off table
• Extension Challenge:
  • Create a claw that can open AND close by pulling different strings (one string opens, one closes)
  • Design a claw with a twisting motion (rotate the cup while holding it)

Safety Notes:

• Scissors should be used carefully
• If using hot glue, teacher supervision required
• Make sure strings aren’t too long (tripping hazard)

Discussion Questions:

• What made your claw work well? What didn’t work?
• How did pulling the string make the fingers move?
• Why was it hard to control the claw?
• Where do you see claws or grippers in real life? (Arcade games, robot arms, construction equipment!)

Real-World Connections:

• Robotic arms in factories use similar gripper mechanisms
• Arcade claw machines use this same principle
• Prosthetic hands work with similar control systems
• Construction cranes have claws for lifting materials

Troubleshooting Guide:

Problem	Solution
Claw won’t open	Add rubber bands to pull fingers apart when string is released
Claw won’t close	Pull string tighter; add more tension with rubber bands
Fingers too stiff	Use thinner materials; make hinges looser
Can’t grip cup	Add rubber bands around fingertips for friction; curve fingers inward
String keeps slipping	Tie knots; add tape to secure string to sticks
Cup slips out	Hold claw steadier; close fingers more tightly; add texture