Can you solve the birthday cake riddle? - Marie Brodsky

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Let’s Begin…

Your friend’s birthday is tomorrow, and he’s turning… well… you’ve forgotten. A ginormous cake has been prepared and your job is to sculpt his age as the chocolate centerpiece. The birthday boy is a giant, and you’re afraid that if your forgetfulness becomes known, you’ll become part of the feast. Can you figure out your friend’s age before the party? Marie Brodsky shows how.


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TED-Ed Animations feature the words and ideas of educators brought to life by professional animators. Are you an educator or animator interested in creating a TED-Ed Animation? Nominate yourself here »

Meet The Creators

  • Educator Marie Brodsky
  • Director Igor Coric, Artrake Studio
  • Narrator Addison Anderson
  • Music Cem Misirlioglu
  • Sound Designer Cem Misirlioglu
  • Director of Production Gerta Xhelo
  • Editorial Director Alex Rosenthal
  • Producer Bethany Cutmore-Scott
Additional Resources for you to Explore
Fun fact: the classic version of this riddle casts you as a train conductor trying to count the number of train cars in a train, where all you can do is walk from one car to the next and turn the light on or off! Here is the puzzle as described by the riddle section of FiveThirtyEight. If you want to dig deeper into the math, Professor Tim Black has a nice blog about this puzzle analyzing the efficiency of different solutions and Ben Tupper has a video walking through several possible approaches.

Puzzles like this one challenge your brain to figure out something new, with no clear ties to problems you already know how to solve! The more of these you tackle, the more equipped you become at adapting to new challenges that come your way. Think back to how impossible this problem seemed at first, and reflect on the “aha” feeling you may have experienced once you realized returning to the starting candle would help. The TED-Ed riddle series is a great place to find more puzzles, each different from the last!

Customize this lesson

Create and share a new lesson based on this one.

About TED-Ed Animations

TED-Ed Animations feature the words and ideas of educators brought to life by professional animators. Are you an educator or animator interested in creating a TED-Ed Animation? Nominate yourself here »

Meet The Creators

  • Educator Marie Brodsky
  • Director Igor Coric, Artrake Studio
  • Narrator Addison Anderson
  • Music Cem Misirlioglu
  • Sound Designer Cem Misirlioglu
  • Director of Production Gerta Xhelo
  • Editorial Director Alex Rosenthal
  • Producer Bethany Cutmore-Scott

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