When I first introduce greatest common factors to the students, we do a Venn diagram activity on the black board with magnetic numbers. I put two collapsible circles on the blackboard (I LOVE these circles - they have so many uses in my classroom) and hand out the magnetic numbers (from 1 to 100). I call out two numbers and the students that have factors for these numbers have to come up and put them in the right spots on the Venn diagram. It's fun to watch them think and talk it out with each other as they decide if the factor is common to the two numbers. I have shared my numbers on google docs. Just click HERE to download a copy. Simply print on magnetic paper, cut out, and you're set! (I use these ALL the time in the classroom, too - I keep them in a ziploc baggie beside my desk).
Hope you're having a relaxing Sunday! We have one more full week and then we're off for a week for March Break!!! YAY!