How do fish breathe underwater?

You ask: How fish breathe water?

Water not just wet — it hold a tiny bit of air-breath mixed right in, the same breath Caveman want. Fish can grab it. Fish gulp water in the mouth and push it out through side-slits. On the way out, water flow over the gills — feathery red combs full of thin skin and blood. Air-breath in the water pass through that thin skin straight into fish blood, while bad gas pass back out into the water. Blood flow the opposite way the water go, so fish grab about eight parts in ten of the breath — far more than Caveman own lungs pull from air, clever trick. Then blood carry it through the whole fish. Cave wisdom: fish not sip air from bubbles — fish pull breath straight out of the water through feathery gills.