Star Turn Bilateral symmetry is one kind of symmetry. It’s not the only kind. In fact, some things have two diff erent kinds of symmetry. To see how that works, dive into the sea.

You’re looking for a pretty sea star with fi ve pointy arms. T ere’s one. It’s crawling across the seaf loor. Now look for the matching halves. It’s not

quite as easy to fi nd with a sea star. You can, though. T e line goes right down the middle of one arm. So each half of the sea star has two whole arms and one half arm. Look at the sea star again. See how its fi ve

arms stick out from its center. T at means the sea star has a second kind of symmetry, too. It’s called rotational symmetry. You can turn, or rotate, the sea star and it looks just like it did before you moved it.

Heads or Tails T e sea star looks the same aſt er you turn it because it doesn’t have a head or a tail. Instead, it has a mouth in the middle of its body. Its arms stick out from that central point. Only a few kinds of animals have rotational

symmetry. T ey all live in the water. Yet you can fi nd plenty of other examples of this kind of symmetry in nature. A passion flower is a good example. Look

at the photo of it. Its yellow petals fan out in a circle from its center. So do the spiky purple and white parts. Now rotate the photo a little bit. You’re looking at the image from a diff erent angle. Yet it still looks almost the same. T at’s rotational symmetry. If passion flowers don’t grow near you, look

for daisies or dandelions. No matter how you turn these flowers, they look the same.

If you pick up this sea star, turn it, and put it down, it still looks the same. That’s a kind of symmetry.

6 NATIONAL GEOGRAPHIC EXPLORER

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