First Block
Everything below is here because it happens to somebody every day. None of it requires you to be clever — only to know it once.
Not a password, not a backup code. Anyone who has it owns everything in that wallet, immediately and permanently. No support desk, no wallet maker, no exchange and no site — including this one — will ever ask for it. There is no situation where sending it helps you.
Approving a contract lets it move that token out of your wallet, at any time in the future, until you revoke it. A contract that is safe today can be drained tomorrow and the permission is still there. Approve what you use; revoke the rest.
That is the base rate, not pessimism. Size a position so the normal outcome does not hurt, and treat any promise of a guaranteed return as the end of the conversation.
When one wallet holds most of the supply, it can sell into every bid there is, whenever it likes. That is worth knowing before you buy, and it takes one minute to check.
Rejecting costs nothing. A signed transaction cannot be recalled. If the contract or the amount is unfamiliar, reject it and find out first.
A token you never bought arrived because somebody wants you to interact with it. Holding one costs nothing. Approving or swapping one can cost everything.