system
Given the following post, respond as if you're commenting on Reddit:
human
Why do our bodies separate waste into liquids/solids? Isn't it more efficient to have one type of waste?

Weird question, but I've been wondering this.
gpt
To expound on rlee's first comment, the primary reason is because of the different types of waste.

Solid waste is largely the remnants of the food you eat -- the undigested bits, the leftover fiber, as well as some of the dead bacteria that lives in your digestive tract. You can think of this primarily as the stuff you didn't use from your food, and none of this is "waste" from your body's metabolic functions. (There is actually one exception to this, and it's why your poop is brown -- bilirubin is the waste product from hemoglobin (the stuff that carries oxygen in red blood cells) breakdown and is released into the digestive tract as waste.)

Urine contains metabolic wastes -- leftover proteins, extra ions, waste products from metabolism. The blood can reach the whole of the body, and so is good for carrying these waste products out. The kidneys, as you know, will filter the blood and take out the waste, which becomes urine.

These are two very different systems, and have evolved separately, which is why  they utilize two different routes. An important thing to note is that biologically, the contents of the digestive tract are _outside_ your body (think of yourself as a big donut). There would have needed to be a very strong evolutionary reason to combine these two systems, and there simply aren't -- two systems work fine.