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I am currently working on source material for a math-related software project with my mother, who has a PhD in Elementary Education and specializes in math education. While she has quite a strong fundamental understanding of the current techniques and topics taught to children at an early grade school level, I am relatively unfamiliar with the subject area. Lately, I've been looking for sources of inspiration and information, such as Conrad Wolfram's TED talk "Teaching kids real math with computers".

Remembering that the Stack Exchange family now has this excellent math site, I figured I would ask this question here:

What are some interesting concepts in math that can be taught to elementary school children, that aren't traditionally taught? This can be because the topics aren't considered suitable, or perhaps because no one's thought to do it yet. For example, at the end of his talk, Wolfram describes a technique to visualize calculus using limits and shapes inscribed in a circle.

(While I realize this question is slightly unusual compared to other questions I see on the front page -- in other words, university-level math questions -- I think it is appropriate given what the FAQ mentions as appropriate topics for this site.)

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I think you might get more useful answers if you provided more specific information. First, if you are talking about American students, please be explicit about this: a lot of the readers of this site come from other educational cultures. Also, to me "elementary school" means students aged 6-11: is that accurate for you? This still leaves quite a range: are you looking for activities which would be appropriate across this range or would it be okay to have some activities which are better geared towards older (or younger?) students? –  Pete L. Clark Dec 29 '12 at 9:20
    
Finally, are you looking to devise materials for particularly bright / gifted / motivated students, or materials that would capture the attention of a wide range of elementary school students? There is a very small percentage of children who are ready for truly advanced mathematics at a very young age; however the majority of students are not. –  Pete L. Clark Dec 29 '12 at 9:22

3 Answers 3

I've had success teaching an elementary school student about binary numbers. I listed powers of two up to 512 and claimed I could make any number up to 1000 by adding the powers of two, using each one at most once. I had the student give me numbers and demonstrated how to make them out of the powers of two.

Then I had the student do it herself for some small numbers using guess-and-check. Then we made a table with powers of two as columns and sequential integers, from 0, as rows, and put an X in any entry if the power of two for that column was used to make the number in that row.

We looked for patterns in the X's, and were introduced to binary numbers that way. By the end of two hours, she could convert any number between binary and base ten, and could add and multiply in binary.

I haven't tried to do it and don't know how, but I suspect you can introduce basic group theory to students of this age, too.

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Prologue

I am $13$ years old, which is neither advanced nor elementary. In this post, I'd recall the mathematical concepts that fascinated me a lot in the past years.


A good thing to learn about is elementary number theory. I learnt how odd numbers are in the form $2k +1$ and even numbers are in the form $2k$, where $k$ is another integer. It was really interesting to tackle the proofs related to this. The realization came to me that remainder is not just another number when I studied modular arithmetic. It was a whole new experience. One is never taught elementary number theory throughout the elementary school (and mostly high school) years!

Besides that, I learnt about number bases and how to convert them. I also watched the TED talk the OP shared succeeded by buying Wolfram Mathematica. It's a great software and I use it for manipulations I couldn't do by hand. Also, I used my resources a lot: MIT OCW, Khan Academy, OpenStudy et al.

Teaching an elementary MathJax is enough for him to go to math forums and ask (So did I!). A little remark from my side: It'd be really, really good to have a site like Math StackExchange for juniors, where a little leniency is shown to newbies.


Epilogue

The above post is self-centered (sorry!), but the things I mentioned are good enough for any other elementary.

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The following is a list of mathematical ideas that I successfully explained to kids (10-12 or so). I list the topics using the formal terminology but, of course, when explained to young kids I usually employed a more intuitive, vivid language and setting. Some of the following took several hours over some sessions to explain. Other are a lot easier.

  • Proving that there are infinitely many prime numbers. The standard proof (basically, multiply a bunch of primes, add one, look for a prime divisor, viola: a new one) requires very little facts about divisibility that are easily explained. The entire logical sequence of the proof is rather short. A preliminary introduction to primes, some examples, and then the question: "how many are there" are usually quite well appreciated.

  • Proving that the cardinality of the even numbers is the same as all naturals. A preliminary discussion of how to compare finite sets without counting easily leads to the notion of bijection as the means for comparison. Then, listing the evens and the naturals in a bijective form is very illustrative and easily digested.

  • Proving that a countable union of countable sets is countable. This may be quite a bit trickier but doable. Starting off with union of two countable sets is countable is a good idea. Of course, the whole discussion can be placed at the Hilbert Hotel making things more vivid.

  • Proving that $[0,1]$ is uncountable. The way I usually approach this is, instead of using the real numbers, to introduce into the Hilbert Hotel a kid into each room. Each kid has an infinite countable list of his/her toys (where there are only two kinds of toys in the world). Then showing that it is impossible that all kids with all possible lists of toys reside in the hotel using the standard diagonalisation.

  • Proving that $\sqrt 2$ is irrational. I managed to do that successfully only one with a young kid so maybe it's too advanced.

  • Proving Sperner's Lemma. Here is the proof I use: Draw a triangle with a triangulation of it. Draw numbers on the vertices according to the statement of the lemma. Treat each small triangle as a room, with the edges as doors with locks of type 1-2, 1-3, or 2-3 according to the numbers on the vertices. Declare that any room numbered 1,2,3 has a great party going on in it. The kid is looking for the party and is given a 1-2 key. Convince the kid that a way into the complex exists along the long 1-2 edge. Use the key to go in, and explain that a door seals itself forever after you go through it. Then either you found a party or you can get out of the room. Then either you are out of the complex, in which case you can go back inside, or you found the party. Keep going until party is found.

  • Pythagoras' Theorem (with at least 3-4 different proofs). There are so many accessible proofs I won't say more.

  • Exploring some ideas of spherical geometry (no parallel lines, exploring sums of angles of triangles etc.). I once sat down with a young girl and saw she got really bored with the school math I was helping her with. So, I took an orange and carved some straight lines onto it and started asking her questions about it. Before long she was going through the entire bowl of apples and oranges carving all sorts of things on them.

  • Constructing an approximation of the hyperbolic plane by cutting several identical paper annuli, chopping them down into reasonable pieces and gluing them together allowing the curvature to take effect. Then, explore lines on the model. Easy and fun.

  • Playing with a Moebius streep: at least cutting it along the middle and then cutting it along 1/3. Easy and fun.

  • Playing with identification spaces. Listing all those obtainable from a square, thus including the Klein bottle. Easy and fun, especially if you explain that the identifications are actually done by using tele-porters.

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I am skeptical that most of these topics would be suitable for most elementary school students (i.e., children aged 6-11). Most of your topics are proofs, but if experience (and literature/theory on cognitive development) is any guide, most children of that age are simply unprepared to reason abstractly. By way of comparison, I first saw the proof of the irrationality of $\sqrt{2}$ at the age of about 14-15, and I remember being underwhelmed by it: assuming that a fraction was in lowest terms and showing that it then couldn't be seemed like being convicted on a technicality. –  Pete L. Clark Dec 29 '12 at 8:58
    
It wasn't until I was about 16 that I appreciated proof-based mathematics for its own sake. (And this is a story told by someone who became a theoretical mathematician.) Looking back at the irrationality of $\sqrt{2}$, I can't help but think that some other presentation besides the standard one might have made things more exciting. In general, I think that in capturing the attention of those who are not already sold on mathematics, effective presentation makes all the difference... –  Pete L. Clark Dec 29 '12 at 9:01
    
As I said, some of these topics are more suitable for young adults. However, I did have the pleasure of discussing some of these issues very successfully with young kids (10-12). In particular, exploring spherical geometry, infinitude of prime numbers, Pythagoras' Theorem, counting principles, some aspects of cardinalities. I have no idea what the cognitive development books say but the magic word 'proof' in mathematics is nothing but a synonym for 'explanation' or 'reasoning'. Kids are perfectly happy with explanations. –  Ittay Weiss Dec 29 '12 at 9:03
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You are absolutely right! I will shorten the list. Thanks! –  Ittay Weiss Dec 29 '12 at 9:19
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Ittay: no problem. I'm glad that you took my constructive criticism in the spirit it was intended. Obviously you are trying to help, and I'm trying to make your response more helpful. –  Pete L. Clark Dec 29 '12 at 9:23

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