We take 1 block, 2 blocks, 3 blocks, 4 blocks, … all the way to 12 blocks, and see which number can be made into different flat rectangles.
Supplies: 12 cubes or blocks of the same dimension What to do: Give your child a number of blocks (I like to go in order and do 1, then 2, then 3, etc) and invite her to make different flat rectangles. The ones that can only be made into one long rod are called "prime numbers".
How it went
Finally, I found an activity suitable for Kindergarteners. Both kids had fun and, when they learned about prime numbers, they started saying “Oooh, PRIME number” (like “Oooh GHOST”).
The kids tried to make two columns with each group. It was a little strange to start with 1, 2, and 3. They made the same rectangle sideways such as 1×2 and 2×1, which I told them we wouldn’t count as a different size (technically, one could count them as different but I didn’t have enough blocks for the higher numbers).
When we reached 7, Bel told me she found a pattern (that it was possible to make a different rectangle with every other one – she never used the word “odd” and “even” though). I asked her to wait a little bit so we can confirm the pattern. They had a bit of difficulty with 9 because they tried to do two columns and didn’t think about three columns so I had to help a little bit: I gave Nia two columns of three blocks each and there were three stray blocks left for her. We went all the way up to 12.
Note: It was a little awkward to just say “1 is not a prime number – we just decide it is not because it is just itself”.
Explanation: most mathematicians will say 1 is not a prime number. The reason is because we want to say “There is only one way to decompose a number into its prime factors”. It 1 were a prime number, 4 could be written as 2×2, but also as 1x2x2, or 1x1x2x2, etc.