Math Magic Trick

Want to show off a little 'magic' in math class tomorrow?  Want to watch your students' jaws drop when they see you guess their secret number correctly again and again?  Let me teach you how to do the coolest (and simplest) math trick around!

I think you're really going to love this one.  It's easy to prepare and so much fun to do, even as adults.

Here's how it works:

1. Click here to download a free PDF of the Magic Math cards. Print out the Magic Math cards page and cut out each of the cards individually.

2. Place all 6 cards face-up in a pile and put the "Pick a Number Between 1 and 30" card on top.  The order of the other 5 number cards does not matter.

3. Show your student the "Pick a Number Between 1 and 30" card and ask him/her to pick a number between 1 and 30.  Be sure that the student does not tell you what it is.

4. Show the student the first number card in the pile and ask, "Is your number on this card?"  Be sure that the student looks carefully before responding.

• If the student says "yes," make a mental note of the number in the top lefthand corner of that card (it's either 1, 2, 4, 8, or 16).

• If the student says "no," just continue.

5. Show the student the next number card in the pile and do the same as you did before.  

6. Repeat until you have asked the student for all 5 number cards.

7. You should have kept a mental note of the top lefthand corner numbers for all of the cards to which the student responded "yes."  Add those numbers together.

• TIP: Keep a running total in your head as you move through the cards instead of waiting until the end to add them. It's much easier to remember a running total than to remember a bunch of different numbers.

8. The sum that you just calculated is the student's secret number!  Now's the fun part - blow the student away by sharing what his/her secret number is!

How cool and simple is that trick?  You can repeat it with your students again and again, but I guarantee that they will quickly want to learn how to do it themselves...

Getting students to figure out how it works:

There's the old saying that "magicians never tell."  I'm more of the believer that magicians never tell without making their students struggle first.  I love using this trick to get students thinking; it makes quite a challenging small group discovery activity.  Plus, I find that students really get into it because they truly want to learn how to do it themselves.  Here's how I generally scaffold the activity, in case you're curious:

• I give each group a set of their own cards and let them struggle within their groups to re-create what I already showed them. I don't give any "hints" at this time.

• After some time, I direct students to create a chart that shows the expected yes/no card responses to each possible secret number starting with 1. This usually leads them to come up with lots of interesting ideas about how it works, though rarely is it the correct one.

• Once they've generated many wrong ideas, I give them the hint that the secret has something to do with the top lefthand corner number on each card. Because they've had time to create a chart that shows secret number vs. yes/no response on card, I usually have one student who's able to figure it out. If not, I keep giving "hints" until we're able to discover how it works as a class.

Once we've all discovered how it works, I always give each student their own set of the cards so that they can practice it themselves (this will undoubtedly be their favorite part).  I tell them to show their magic trick to their friends in other classes, their family members, their neighbors, etc.  It's a great way for them to practice mental addition calculations through 30... and they won't even realize they're practicing!

Did your students enjoy learning this magic math trick? We'd LOVE it if you'd share one of these images on social or leave a comment in the space below!