This "Science Note" looks at the way that the shapes of simple molecules can be explained in terms of the number of electron pairs in the valence shell of the central atom. This theory is formally known as valence shell electron pair repulsion (VSEPR) theory. The article explains the preferred shape of chlorine trifluoride (ClF3), corrects an area of confusion in pre-university texts regarding lone pairs, and identifies and explains examples of where VSEPR theory fails. (Contains 5 figures.)