Excretion is the process of removing waste materials from the body. Every living thing, from bacteria to blue whales, produces waste as it turns food into energy, and excretion is how organisms get rid of what they can't use.

In humans and animals, excretion happens in several ways. When you urinate, your kidneys are excreting waste products filtered from your blood. When you exhale carbon dioxide, your lungs are excreting a waste gas your cells produced. Even sweating is a form of excretion, releasing salt and other substances your body doesn't need.

Plants excrete too, though differently than animals. They release oxygen as a waste product during photosynthesis (which is lucky for us, since we breathe it). Some plants excrete excess salt through their leaves or roots.

Without excretion, waste would build up inside living things and poison them. Scientists who study how bodies work pay close attention to excretion because it reveals how healthy an organism is and how well its systems are working.