Q. What is Nachas?

Last Shabbos, when my three-year-old grandchild overheard his parents discussing his imminent entry into Citah Beys (2nd grade) of Kindergarten and their hopes of seeing him graduate from Citah Tet (9th grade) as a Bar Mitzvah boy. He sat there thinking and then piped up: "when I will be a real large Bochur (Bachelor), I'll be in Citah Shin (the letter before the last in the Hebrew Alphabet)...