An organism is any living thing: a complete, individual life form that can grow, use energy, respond to its surroundings, and (usually) reproduce. You're an organism. So is your dog, the oak tree in your yard, the mushroom growing near its roots, and even the tiny bacteria living in your stomach that help you digest food.

What makes something an organism rather than just a collection of parts? An organism works as a unified whole. A heart by itself isn't an organism because it can't survive or function independently. But a mouse is an organism: all its parts work together as one living system. Even single-celled organisms like amoebas count, because that one cell does everything needed for life.

Scientists classify organisms into broad groups: animals, plants, fungi, protists (mostly microscopic organisms like algae), and bacteria. Some organisms are incredibly simple, like bacteria that consist of just one cell. Others are fantastically complex, like blue whales or redwood trees. Despite these differences, all organisms share the basic characteristics of life: they grow, need energy, respond to their environment, and maintain themselves as organized living systems.