system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: what is a 401k and how does it work?
gpt
It's a retirement investment account. You may put a portion of your income into it tax-free (ie, you don't pay taxes on the portion of income you put into the 401k up to a limit). You are able to invest the money in the account in a selection of mutual funds to theoretically allow the amount to grow instead of simply being held there. Upon retirement, you may withdraw from the account as normal income (so it does get taxed at that point).