Requests for Admission
From LegalLanding
FRCP Rule 36 allows any party to send requests to any other party asking for an admission of the truth of any matter within the scope of FRCP Rule 26(b). The responding party is deemed to have admitted the truth unless he denies them, and denials must “fairly respond to the substance of the matter.”
This discovery tool is very inexpensive, although drafting requests might drive the cost up. You have to be able to write the question in an airtight way so the only ways to answer are to admit or deny. If the other party denies without having good reason to do so, then huge sanctions will be levied (see Sources of Sanctions). If the other party admits, then it is like doing so in the pleadings and can only be changed by amendments to pleadings.
A good use of this tool is to attach documents and have the other side admit they are authentic. As such, this is not much of a discovery tool. It is good for pinning down information you already have, not getting new information.