Patents grant a limited monopoly for a term of generally 20 years from the filing date, giving the patent holder the right to exclude others from making, using, offering for sale or selling the invention in the United States, or importing the invention into the United States. In exchange for this limited monopoly, the patent applicant must describe the details of the invention in the patent, which becomes a public document. Thus, after the 20 year term expires, the public is free to practice what was formerly protected by the patent without incurring liability for infringing that patent. (Note: it is possible that practicing what the patent describes could still infringe another non-expired patent, so one is not necessarily free from patent infringement by virtue of one patent expiring.) One of the requirements of patentability is that the applicant must disclose the “best mode” known for carrying out the invention. I.e. – she cannot leave the secret sauce out of the patent. Failure to comply with this statutory requirement renders the patent invalid.