Symmetry (noun, “SIM-meh-tree”)
A shape with symmetry has parts that match each other in some way.
Imagine cutting a heart from a piece of paper. To ensure the two halves match perfectly, you might fold your paper in half. Then, you can cut both sides of the heart at once. When you unfold the paper, the two sides are identical. The heart is symmetrical.
The line down the center of the heart, left over from folding your paper, is called the line of symmetry. Folding a shape along a line of symmetry matches up the parts. This kind of symmetry, across one line down the middle, is called mirror symmetry. That’s because the two sides of the heart are shaped like mirror reflections of one another.
But symmetry can come in different forms. Rotational symmetry is another example. This describes a shape that you can spin into a new position that looks identical to the first one. A propeller often displays this kind of symmetry.
Symmetry appears in nature. The elegantly branching crystal shards of a snowflake often have rotational symmetry. Leaves of a tree may display mirror symmetry. The body shape of a cat or dog or person also shows this kind of symmetry. When referring to living things, biologists often use the word bilateral symmetry. It means the same thing as mirror symmetry.
Radial symmetry is another example of symmetry in body shapes. This describes a body shape where you can draw several different lines of symmetry that cross the center. Jellyfish and starfish are examples of creatures with radial symmetry.
Not all living things are symmetrical. A marine sponge usually lacks matching parts. So, this type of body plan is described as asymmetrical.
In a sentence
Floral designs on ancient pottery suggest people understood symmetry 8,000 years ago.
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