After completing a circles unit, I found this great circle activity on http://www.3rdgradegridiron.com/2012/05/want-to-have-blast.html. I made a bubble mixture using the following recipe: http://chemistry.about.com/od/bubbles/a/Bubbles-That-Dont-Pop.htm. I poured the mixture into a plastic plate for each student and then the students blew bubbles by blowing into straws. After they formed a bubble, they found the center, measured the diameter, and calculated the circumference of each bubble. I loved this lesson because they not only had fun, but they also were able to apply what they learned!