"trick and treat" is OSTER project's first Rin and Len duet song. It was uploaded for Halloween 2008. OSTER project's works are usually cheerful-sounding, but "trick and treat" was her first work to have a darker atmosphere. The song reached one million views on November 21, 2011, making it the second Kagamine duet song ("Gekokujou" being the first) to do so. "trick and treat" basically tells a story of "you", who was led by Rin and Len into a deep, deep forest. Although they seemed friendly at first, "you" realized things were not as they seemed.