When dealing with sponges, that used to be the question. Finally, someone figured out that sponges have special cells that perform functions common to animals - they breathe, eat, and expel wastes. So, sponges were classified as animals, rather than plants or anything else.
Sponges are all in the Phylum Porifera - an appropriate name, since the word porifera comes from Latin words meaning "pore-bearing." Sponges have a series of pores and canals through which they bring in, and expel, water for respiration, feeding and getting rid of wastes.
It's their extraordinary ability to hold water that drew us to sponges - they are commonly used for cleaning, although now most of us use versions made of wood pulp or foam. They are also used for bathing and painting.
Although the natural sponges you find at most stores are a brownish-yellow or light cream color, real live sponges can be much more colorful - shades of red, orange, yellow and purple are common, as you can see in the image here.
Image: A variety of sponges seen on a NOAA expedition in the Cayman Islands in 60 feet of water - yellow tube sponge (Aplysina fistularis), purple vase sponge (Niphates digitalis), red encrusting sponge (Spiratrella coccinea), and gray rope sponge (Callyspongia sp.). Images Courtesy of the Twilight Zone Expedition Team 2007, NOAA-OE
There were some good guesses to the most recent 