Why isn't algae a animal?

Answer 1

Big Picture answer:

Animals are generally distinguished from plants by being unable to synthesize organic molecules from inorganic ones, so that they have to feed on plants or on other animals. They are typically able to move about, although this ability is sometimes restricted to a particular stage in the life cycle. And they are commonly defined as being able to respond quickly to stimuli (though that is somewhat relative). Algae fails to meet these requirements.

Answer 2

Algae are not animals because they belong to a different kingdom known as Protista or Plantae, depending on the type of algae. Algae are simple, photosynthetic organisms that lack the specialized tissues and organs found in animals. They also have cell walls made of cellulose, which is a characteristic of plants rather than animals.