The gold bar is a line of symmetry. Whatever you paint on one side is copied to the other — the same distance from the line, but flipped, like your face in a mirror.
Slide or spin the line and the copy follows at once, because the line itself decides what counts as "the same on both sides." Paint half a shape and the mirror finishes it for you.
Cross a second mirror over the first and the copies multiply — your one stroke repeats around the middle. That is rotational symmetry, the pattern inside every kaleidoscope.
Snowflakes, flowers, butterflies, and tiled floors are all built from these same simple mirror rules.
Something in the easel stopped unexpectedly — the lesson continues without it. You can move on; nothing you did was wrong.