system
Given the following post, respond as if you're commenting on Reddit:
human
What's the difference between lungs and gills that makes one liquid friendly and the other gas friendly?

aka Why can't i breathe water god damn it, what is stopping me? dumb question i know, just uneducated and curious.
gpt
Lungs and gills are fundamentally similar on the very smallest scales.  Blood vessels run near to a fluid (either air or water) which contains oxygen.  O2 comes in, CO2 goes out.

But on large scale, there are important differences.  To minimize water losses, lungs are essentially bags (well clusters of many many tiny bags) which expand and contract, drawing air in and forcing it out.  This works because air has very low viscosity.  Water would get stuck down in the lungs, and would have a much harder time for new water to diffuse into all the nooks and crannies with each breath.  Also, mechanically you would need more muscle power to draw in and squeeze out the water.  Additionally, air contains much more oxygen than is dissolved in water.  There's just not enough oxygen in a lungful of water to meet your needs as an active mammal (I don't know this for sure but it's probable).

Gills, on the other hand are completely different.  Essentially they are rows of filaments which have water flowing over them constantly.  They can't work in air for two reasons.  First, if they are taken out of the water the filaments collapse like hair on a wet cat.  This dramatically drops the surface area available for gas exchange.  Second, the cells are unprotected in a chamber, and are thus vulnerable to dehydration.  Lung cells are always pretty humid thanks to their internal location, but gills will dry to a crisp and the cells will die.