The etrog (citron) is a citrus fruit that is used by Jews every year during the holiday of Sukkot (late September, early October, depending on the lunar calendar). It's entire purpose during the week-long holiday is to smell nice. Yes, that's it, we just smell the thing. After the holiday is over you can chop up the etrog and make jam. What I like to do is to stud it with cloves and then use it as the most aromatic besamim for the havdalah service signifying the end of the Sabbath. Aside from the etrog and the cloves, you will need a thin crochet hook to pierce the fruit before sticking in the clove. I use a 0.75 hook, but a 1.00 will work just as well. These babies last for years, and if you already have one or two (or ten or eleven, as is the case in my house), they make lovely gifts. BTW, you can also use a lemon or an orange, the results are the same, just shaped differently.