Sal introduces the concept of an axis of symmetry.167
Axis of symmetry is a free educational video by Khan Academy.It helps students in grades 4 practice the following standards 4.G.A.3.
1. 4.G.A.3 : Recognize a line of symmetry for a two-dimensional figure as a line across the figure such that the figure can be folded along the line into matching parts. Identify line-symmetric figures and draw lines of symmetry..