The esrog (or etrog) is one of the four traditional trees used in the rituals of the seven-day holiday of Sukkot (which is a bit like the Jewish Thanksgiving). An esrog fruit looks like an elongated lemon, and the trees are used in the special prayers said during that holiday. An ideal gift for Sukkot itself, an esrog tree also makes a wonderful houseplant. They can grow indoors and will thrive even in a small apartment.