system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5 Why is kelp not classified to be a plant but moss is? Kelp seemingly have more plant parts but it is classified as a algae, but moss is a plant?
gpt
Down at the cellular level, algae and plants are different. There's a ton of confusing overlap so it's not really clear cut but, basically, although kelp has parts that "look plant-ish" like "stems" or "leaves" or "roots", if you look at the individual cells that make up those parts kelp and plants look really different. 

Meanwhile, although moss might not look much like a palm tree, they both have stems and leaves and roots and if you get down at the cellular level they're basically the same types of cells. 

Basically, plants are using one set of building blocks and algae are using another. Imagine plants are Lego...you can build all kinds of sizes and looks of Lego models but the individual building blocks are all basically the same. Algae are some other building set that's not Lego-compatible, let's call that Algo...you can also build all kinds of sizes and looks of models out of Algo, but you can't connect Algo to Lego.