system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why are most lids not recyclable, but their containers are?
gpt
The lids usually are as well. Unfortunately, although it is standard practice to mark the container, it is not as common  to mark the lid with what plastic it is made from. Mostly the lids are made out of the same material as the container, but this is not always the case. For example: yogurt containers are usually made from polypropylene (PP), while the lids are usually made from polyethyleneteraphthalate (PET).

Plastics may not be compatible with each other when they are reprocessed. So if the plastic is unknown, it must be avoided to stop contamination to other known plastics.

This means that although the lids could be perfectly recyclable, they will often have to be discarded to landfill. Yes, this is a stupid system. It could easily be avoided by requiring all lids to be marked as well as the container.