Y: Hey Don, how would you like to hear about an animal stranger than most science fiction aliens?
D: What animal is that, Yaël?
D: Sponges? You must be kidding. I use sponges to clean my kitchen.
Y: Oh, Don, those usually aren't even real sponges. Cleaning sponges today are typically made from cellulose or plastic. Real sponges are living creatures. Cleaning and bath sponges were originally made from their flexible protein skeletons.
D: So, then, what really are sponges?
Y: Sponges are simple aquatic animals. Their bodies are like balloons full of jelly. The "balloon" is made of layers of cells. Others cells inhabit the jelly and produce its skeleton.
D: But, sponges are anchored in place and don't move. That seems more like a plant than an animal to me.
Y: Only adults are mostly stationary. Their larvae move by swimming or crawling. Plants feed by capturing solar energy in the process of photosynthesis. Sponges can't do this, although a few species host cyanobacteria that can.
D: If they can neither move nor photosynthesize, then how do they eat?
Y: Sponges pump water through the hollow canals of their porous bodies, using microscopic beating hairs on special collar cells lining the canals. The flow carries particles of food, and oxygen too, into the sponge to be captured by its cells.
D: What about sex? If they are animals, there must be males and females.
Y: Umm, well, like other animals sponges produce sperm, which can fertilize egg cells in another individual. But, each sponge can function as both a male and a female.