On a sheet of paper, draw a grid that is 10 squares across and 5 squares down. Write the numbers from 1 through 50 in the squares. The numbers 1 through 10 should be the first row, the numbers 11 through 20 should be the second row, and so on.

Begin with the number 1. The number 1 is neither prime nor composite, so draw a slash through 1. Next, consider the number 2. Use the grid paper to shade as many differently shaped rectangles as you can that contain 2 square units. You should find only 1. Since only one rectangle can be drawn for 2, it is a prime number. Do not draw a mark through 2. Now repeat the process of shading rectangles for each number from 3 through 50. If the number has more than one rectangle, it is composite. Make a mark through each composite number. Continue drawing rectangles until you reach 50. The numbers left on your grid should all be prime numbers.

Were you able to eliminate some numbers without drawing rectangles? If so, why were you able to eliminate them?

Next, one group member should write down any three-digit number. The other group member should determine whether the number is prime or composite by drawing rectangles. Take turns writing numbers and drawing rectangles.