To see what bilateral symmetry is, paint a picture. Take a sheet of paper and fold it in half, carefully lining up the corners. Unfold the paper, and paint a picture just on one side of the fold line. While the paint is still wet, fold the paper again so the paint squishes on to the other side of the fold line. Carefully unfold the paper and admire your bilaterally symmetrical painting! and let it dry.
Or, if you want to be all modern about it, open a painting app and turn on mirroring, and paint on one side while the same thing magically appears on the other side!
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