We discovered that it's fun to draw with two colors at the same time. (And sometimes we even go off the paper and onto the table. Oops!)
When you turn the cup like a propeller, you get a circle picture.
Each child investigated the propellers differently. (I didn't get many pictures of the kids using them. My camera battery was depleted. Boo.)
Here's how we made the propellers: Cut two slits in a foam cup. Slide a large craft stick through the slits. Tape a marker to each end of the craft stick. And make sure the marker ends are at the same level.
(We're using LifeWay VBS curriculum and the idea is from there.)