system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is water so difficult for 3d animation?
gpt
Water presents a number of challenges. First off, since it's a liquid it can move in a huge variety of ways, which means it's difficult to simulate well, and the more accurate you want to get, the more computational time it's probably going to take. If you're simulating something where the water is constantly moving (a river, or waves on a shore), that's a lot of work to be doing if you want it to look really good. A common method of doing this is having the computer represent the water as a whole bunch of spheres that collide with each other and with the surrounding geometry. This determines where the water is going, and then graphics shaders render it to look like a continuous liquid rather than separate spheres. The more spheres you add, the more fine grained and realistic your water moves, but the more you increase the computational requirements. 


Second, water is mostly transparent, but not entirely. So for it to look good, you want to be able to see through it, but not have it be completely clear. If there's enough water, it eventually absorbs a significant amount of light, and not equally across all wavelengths. So you have to try to get that right. Also if you're looking into the water from above it, you get refraction, and if the surface of the water is moving and/or has waves/ripples/etc. then that's even more complicated. 

Third, while being transparent, water is also reflective. So there's another layer of complication, and again it gets even crazier if the surface of the water is in motion. 

Now, a lot of this stuff can actually be simulated pretty realistically, and we've figured out ways to make it look good. But the better of a job you want to do, the more work the computer needs to do, and if you're playing a game there's only so much processing time that the computer can spend on water for each frame. 

So a lot of times various tricks/shortcuts/etc. are used to make something that looks good enough that we'll accept it as water, even though it's not a particularly accurate simulation.