Elfix live ten times faster than human beings. They are more difficult to see than a mouse that runs fast. This is one of the reasons why most humans almost never see them. The other reason is that humans are very good at not seeing things that they think do not exist. And, since reasonable men think that there are no such things as four-inch-tall people, an Elf who does not want to be seen won't probably be seen. Elves are about two spans tall, but they can extend or stretch their shape until they look giant.