system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What exactly makes something a vitamin and how did they get their names (C, B6, B12, E, etc)?
gpt
Vitamins are small organic molecules that we need in tiny amounts in our diet. So for example amino acids are not vitamins because we need large amounts of them. Sodium is not a vitamin because it is inorganic. 

As for the name, basically they were lettered in order we realised their existence. However knowledge was incomplete (and may still be incomplete) so it turns out that what we thought was one single vitamin (B) was in fact a complex of several closely related substances, so those got numbered. Some of them were originally vitamins G-J, hence the sudden jump to vitamin K. Others were found to be non-essential and dropped.