A euglena is a tiny single-celled organism that lives in ponds and streams and can do something remarkable: it acts like both a plant and an animal. During the day, when sunlight hits the water, a euglena uses its bright green chloroplasts (the same structures that make plants green) to make its own food through photosynthesis, just like a plant. But when it's dark or food is scarce, it can take in nutrients from its surroundings, a bit like an animal eating.

You need a microscope to see a euglena, which looks like a miniature torpedo with a long, whip-like tail called a flagellum that it waves around to swim through the water. Near its front end, it has a light-sensitive spot called an eyespot that helps it swim toward sunlight so it can photosynthesize.

Scientists find euglenas fascinating because they blur the line between the plant and animal kingdoms. In fact, for a long time, biologists argued about whether euglenas were plants or animals. Today, they're classified in their own group called protists, along with other organisms that don't fit neatly into the plant, animal, or fungus categories. When you look at pond water under a microscope in science class, those tiny green swimmers zipping around are often euglenas.