ELY5: Fish have special lungs called gills, which are like a stack of delicate tissue paper, with many layers of thin tissue all sitting next to each other. The surface of each tissue paper needs to not touch its neighbours in order to absorb oxygen from the water. If you soak a stack of tissue paper in water, and then take it out; it will be impossible to peel the tissues apart. This is exactly what happens to fish gills once they leave their water home.