In Negaim 11:5, the mishna lists one of the preconditions for burning a garment: if its green or red discolouration hasn't changed for two weeks. Two mishnayot later, in 11:7, Rabbi Eliezer is asked why a garment that doesn't have room for the discolouration to spread would still need monitoring, and he doesn't know the answer. Rabbi Yehuda ben Beteyra tells him that it is because the discolouration might remain for two weeks, in which case the garment would need to be burnt, and Rabbi Eliezer praises him for being a great scholar (חכם גדול אתה).
What is so impressive about Rabbi Yehuda ben Beteyra's answer? Did Rabbi Eliezer not know the law recorded in Negaim 11:5?? I find that rather unlikely, since it's also recorded in Leviticus 13:55. Seeing as it is recorded there (and therefore not a particularly profound observation), why did Rabbi Eliezer not say it himself, and why was he so impressed when Rabbi Yehuda ben Beteyra did?