Fold a piece of paper in half. Cut a shape out of the folded edge. Open it up — the result is symmetric about the fold line. Reflection symmetry, made physical.
Try this
Fold once for one line of symmetry. Fold twice (perpendicular) for two lines of symmetry — you'll get something with four-fold reflection. Fold three or four times for snowflake-like shapes.
Curvy cuts make stranger results. Try cutting a wavy line, then unfolding — you'll get a perfectly symmetric blob you couldn't have drawn freehand.