system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Why it's called a semi truck.

If that's semi, what's a full truck look like?
gpt
The "semi" doesn't refer to the truck. It's called a semi truck because it's built to carry what's known as a semi-*trailer*: a trailer which doesn't have front wheels on it, because it just slides on top of the truck. (There are full trailers that do have front wheels, but they're much rarer.)