You probably know more about the idea of "center of mass" than you suspect. Consider the following pictures of seesaws. Which make sense, and which don't?
Image courtesy of Declan McCullagh Photography
Image taken from G. P. Quackenbos A.M. A Natural Philosophy: Embracing the Most Recent Discoveries in the Various Branches of Physics, and Exhibiting the Application of Scientific Principles in Every-day Life (New York: D. Appleton and Company, 1859)
Image courtesy of EFTrends
Copyright © Michael Richmond. This work is licensed under a Creative Commons License.