image credit: Eleni Katifori; image source; additional image

**Why Leaves Aren't Trees**

This is a lemon leaf that was wounded to interfere with its circulation. Note how the network of veins enabled nutrients to flow beyond the wound.

- Click on the image to see a similar experiment with a ginkgo leaf.

- To learn about the controversy over how these different vein structures developed, see Why Leaves Aren't Trees.

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**Konigsberg Bridge**

If you enjoy solving puzzles, try the seven bridges challenge.

- This problem helped lead the mathematician Leonhard Euler to develop new mathematical fields, graph theory and topology. See Konigsberg Bridge to learn more.

- Also see the Wikipedia page for variations on this problem.