Week 10| Lock and Key Subtraction - Play Discover Learn 24/7

Week 10| Lock and Key Subtraction

Lesson Objective:

Children are introduced to subtraction through a missing rods alternative. 


  • Small Group Set of Cuisenaire Rods
  • "How the Library (Not the Prince) Saved Rapunzel" By Wendy Meddour
  • Free Activity Page Below
  • An imagination

Lesson Plan

Begin with free play for 10-15 minutes. Ask the students about what they are creating and engage with their discoveries.

Break for story time and read, ​"How the Library (Not the Prince) Saved Rapunzel." I love this book for so many reasons.  Okay, I have love rhyming books, but even more, this is a great character building book.  

Rapunzel is feeling down because she has nothing to prove, but when she gets a job a the library, her passion for learning and helping others is unlocked.  

Story Discussion

At the end of the story, begin with a growth mindset discussion.   What are things that can lock you up from being amazing at something? 

Discuss how serving others can make us happy.   How did helping someone make them feel? Did you learn anything from helping someone?

Discuss how reading unlocks opportunities to grow and learn.  Ask them about something they read recently.  What did they learn?  Did it make them want to learn more?


Once the discussion is over, introduce the Lock and Key exercise.    Tell them you are building a lock and they need to find the missing key to unlock it.  

A lock begins with two rods of the same length with a smaller rod sandwiched in between.  Then, try a variety of rods to see if they complete complete the lock.   Is it too large?  Is it too small?  

In the picture here, the key is a yellow rod, that is the missing rod. ​A purple plus a yellow is equals the length of the blue rod. 

Have the students create their own locks and challenge each other to find the missing key.   You can build more complex lock and keys.  See this post for more ideas. This activity pairs well with the play mats from PDL's Subtraction Lock and Key Exploration.  

Math Journal

Offer students the opportunity to draw their favorite lock and key on a Free Discoveries Journal page and encourage them to include elements of the story. You may want to offer to read the story again while they are drawing.

Extended Exploration

To explore the Lock and Key structure further, check out PDL's Lock and Key Subtraction Exploration.

lock and key subtraction cuisenaire rods