Dry Etrog
A Dry Etrog is pasul. How should one check to see if the etrog is dry? Push a needle and thread through it and see if the thread is moist when it comes out. [If it is a year old, you can assume it is too dry.] Shulchan Aruch Orach Chayim 648:1 Taz: Um, isn't that a nekev mefulash - a hole going through the etrog that makes it pasul? However, nekev that makes it pasul goes all the way through or reaches the seeds. What it is referring to here is pushing it off to the side. We learn the psul of etrogim from treifot of animals. Just like we find a hole in the limb of an animal on the side can be kasher, so too here.