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Indoor activity

Secret Spy Telephone

Build a working telephone from paper cups and string, then send top-secret whispered messages to your spy partner across the room!

Ages 3-10 0-1 hours Education 8/10

Materials

  • Paper Cups
  • Pen
  • Scissors
  • String
  • Tape optional

Illustrated Steps

1

Prepare Your Spy Devices

Poke a small hole in the centre of the bottom of each paper cup using a pen or pencil point.

2

Thread the Secret Line

Thread string through each cup hole from the outside and tie a big knot inside so it can't pull through.

3

Test Your Spy Phone

Walk apart until the string is pulled tight. One spy whispers into their cup while the other listens!

4

Set Up a Spy Mission

Write secret messages on paper. Take turns whispering them through the telephone and checking if they arrived correctly.

What You’ll Create

Agents, your mission — should you choose to accept it — is to build a TOP SECRET communication device! 🕵️‍♂️📞 Using just two paper cups and a length of string, you’ll construct a real working telephone that carries whispered messages across the room. Can your spy partner decode the message before time runs out?

How to Set It Up

Step 1: Prepare Your Spy Devices

Take two paper cups and flip them upside down. Using a sharp pen, carefully poke a small hole in the centre of the bottom of each cup. ⚠️ Adult Helper Needed — The hole needs to be just big enough for the string to thread through. A pen or pencil point works perfectly.

Step 2: Thread the Secret Line

Cut a long piece of string — at least 3 metres works best! Thread one end through the hole in the first cup from the outside, then tie a big knot on the inside so it can’t pull back through. Do the same with the other end and the second cup.

Step 3: Test Your Spy Phone

Each spy holds one cup. Walk apart until the string is pulled tight — this is crucial! The string must be straight and taut with no sagging. One spy holds their cup over their ear while the other whispers a secret message into their cup. Can you hear it? 🤫

Step 4: Set Up a Spy Mission

Create secret code names for each spy. Write a list of “classified messages” on paper — silly sentences, secret passwords, or mission instructions. Take turns whispering them through the telephone. The listening spy writes down what they hear. Compare notes — did the message survive the transmission?

Have fun!

  • 🕵️ Create spy ID cards with code names and stick them on with tape!
  • 📝 Invent a secret code alphabet — A=1, B=2 — and send coded number messages!
  • 🏃 Try stretching the string even further — how far can the signal travel?
  • 🤫 Whisper as quietly as possible — can your partner still hear you?

Why It’s Amazing

  • Sound Science: Children discover how sound travels through vibrations — the string carries sound waves from one cup to the other, just like a real telephone wire! 🔬

  • Communication Skills: Taking turns speaking and listening builds conversation skills and patience. 🗣️

  • Creative Play: The spy theme transforms a simple science experiment into an immersive imaginative adventure! 🎭

  • Problem Solving: Figuring out the right string tension and whispering volume encourages experimentation and critical thinking. 🧠

Pro Tips

For ages 3–5: An adult should poke the holes and tie the knots. Keep the string shorter (2 metres) and focus on the fun of whispering and listening.

For ages 5–8: Let them build it independently with supervision. Encourage experiments: what happens with a longer string? A thicker string? Does it work around corners?

For ages 8–12: Challenge them to build a “party line” — connect three cups to one central string. Can all three spies hear each other?

Secret Pro Move: Try using a metal slinky instead of string for one test — the sound quality is dramatically different and sparks great conversation about materials and vibration! 🌀