What are gills? what purpose do they serve in fish?

Gills extract out oxygen from water and carbon dioxide is carried away by water. With the help of gills fish force the water, they flow past lots of tiny blood vessels. There is a seeping of oxygen through blood vessels wall and carbon dioxide is taken out. With gills water flow through their mouth over the gills and taken out with the helps of gill slits. Because water is heavier  and water moves in one direction.

Explanation:

for example, no new evidence will demonstrate that the earth does not orbit around the sun (heliocentric theory), or that living things are not made of cells (cell theory), that matter is not composed of atoms, or that the surface of the earth is not divided into solid plates that have moved over geological timescales .

idk what happened..you tell