Hundreds Charts? Really?

Yes! I borrowed an excellent lesson from the TERC Investigations series that the kids were, I'll admit, bored with in the beginning, but the end surprised them and started kids trying to prove me wrong. I handed copies of 100's charts to my third grade class (the kind with squares pre-numbered - ten numbers to a row and 10 columns). We started by coloring in the number 10 and all multiples (20, 30, 40, etc.) and then multiples of 9, 8, 7, etc. It took a while, and is a standard chart exercise - except that I've always charted multiples of a number on separate charts. Since this was all on the same chart,we were eventually left with just a few uncolored squares. 

I then told them that those numbers are indivisible by anything other than 1 and themselves and were called PRIME numbers. They didn't believe me because they thought that all numbers were divisible by something. this set off a debate and kids doing math problems on the side to prove me wrong. "Anyone want a calculator to figure those problems out?" I said; a few took me up on it. In a few minutes they were looking perplexed.

"So they can't be divided at all?"

"Right except by 1 and the number you see."

So we made a list of them and looked for patterns. Someone noticed that there are few prime numbers in the second half of the chart than in the first half. We speculated about how many prime numbers would appear in the next group of 100 (101 - 200). I feel like they really understood prime numbers after that.