system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are all external body parts, like eyes or ears, symmetrical along the spine but some internal ones, like the heart, are asymmetrical?
gpt
It has to do with embryology (how the embryo develops into a human).
An embryo starts out as a flat disc with 3 layers of cells and then "folds" in on itself (imagine wrapping a burrito).

The innermost layer forms a tube, which will develop into the internal organs, and the outermost layer forms your skeleton, bones and connective tissues (pretty much everything that isn't your organs)

The outer layer develops symmetrically about the midline. 2 eyes, 2 ears, 2 arms, 2 legs. Interestingly, your brain is also symmetrical about the midline (although the right and left brain do have different functions)

As the inner layer of cells (the tube) it becomes squashed against the walls of the body cavity. This causes the tube to twist and bend as it develops. Eventually different parts of the tube differentiate into different organs. The lungs, liver kidneys and spleen branch off the midline. The gut does a whole bunch of Gymnastics. Interestingly the "start" and "end" of this tube (the mouth and anus) ARE symmetrical.

The heart is a good example of this. It starts out as a symmetrical tube, but bends and folds as it develops, ending up in the 4 chambered configuration of a mature heart.

The reason the outermost layers are bot symmetrical is partly because they are not space restricted like the internal organs.