Describe the levels of biological organization -- cell, tissue, organ, organ system, organism -- and classify real examples at each level.
One cell rarely works alone in a large organism. A tissue is a group of similar cells working together on one job -- muscle tissue is made of many muscle cells contracting together; nerve tissue is made of many nerve cells passing signals together. An organ is a structure built from two or more different tissues cooperating on a bigger job. Your heart, for example, is mostly cardiac muscle tissue, but it also contains nervous tissue (to keep its rhythm steady) and connective tissue (holding it all together) -- three different tissues, one organ.
Zoom out again and organs form organ systems: the heart, blood vessels, and blood together make the circulatory system. Zoom out one more time and every system working together makes one organism -- you. So the full ladder runs: cell, then tissue, then organ, then organ system, then organism, each level built from the level below it.
Mastery looks like: They can place any given biological example at the correct level (cell, tissue, organ, organ system, organism) and explain what distinguishes a tissue from an organ.
Common stumbles: Treating "organ" and "tissue" as interchangeable; assuming an organ system is just a bigger single organ rather than several organs cooperating.