A friend of mine uses this lesson to teach her students about prime numbers, and prime factorization. (Note the lesson plan’s author is also named Mindy!). I’ve tried it with 6th graders and it’s pretty clear cut. Each student gets to work with a different number, creating a good learning experience for all. Here’s the lesson: Math Inquiry Lesson

If you’re not familiar with the Sieve of Eratosthenes, here’s the Wikipedia article.