Trap One: a Cell in Your Body Is Not an Organism
This is the big one. One of your heart cells is not a whole living thing on its own — it can't feed itself, breathe, or survive cut off from the body. It's a part. An organism is the complete living thing, and in your case that's the whole you, with every level running together. The level “organism” is the top of the ladder, not the bottom.
Trap Two: a Tissue Is Not the Same as an Organ
A tissue is a team of similar cells doing one job — muscle tissue is just muscle cells, all the same. An organ is a step bigger: different tissues built together into one structure. The heart isn't only muscle tissue — it's muscle, plus nerve tissue to time the beat, plus connective tissue holding it together, plus a lining. One kind of cell working together is a tissue; several different tissues working together is an organ.
Trap Three: a Unicellular Organism Is One Cell Doing Everything
Here's the bit that bends the ladder. An amoeba or a bacterium is a single cell — and it's also a complete organism. How? Because that one cell does everything a whole body does: feeds, moves, senses, gets rid of waste, reproduces. It has no tissues or organs because it doesn't need them — it's the whole job packed into one unit. So “cell” and “organism” aren't always different levels; for a one-celled creature they're the same thing.
And the Quiet One: Systems Can Overlap
You'd think each organ belongs to exactly one system, but they share. The pancreas helps the digestive system (it makes juices that break down food) and the hormone system (it makes insulin). Your nose is part of the respiratory system and the sense-of-smell setup. The mouth serves both digestion and breathing. So an organ can wear more than one hat — the systems aren't sealed-off boxes, they overlap and lend each other parts.