Etrogs cannot be bruised, broken, or have any holes in them to make them Kosher to use during Sukkot prayers. Many Etrogs also have a very delicate ball like growth on the top of the fruit called a pitom, that if it should break off during Sukkot, the Etrog is no longer Kosher. To prevent it from breaking, many choose to keep their Etrog in a sturdy box with lots of cushioning, often time with wool. Etrog boxes also keep out bugs and little hands of children that could possibly damage the fruit.