Why do some pirates wear an eye patch?

Some assume pirates wore eye patches to cover a missing eye or an eye that was wounded in battle, but in fact, an eye patch was more likely to be used to condition the eye so the pirate could fight in the dark.

It takes an average human eye about 25 minutes to fully adapt from bright sunlight to seeing in complete darkness — if a pirate was fighting on deck in the light, then had to go under the deck to fight downstairs where it is usually pretty dark, that would be a long time to go without being able to see. The eye patch could be used to prepare one eye to see in the dark, so when they would go below deck they could swap over the eye patch from one eye to the other and see with the eye that has already adjusted to low light conditions. This would allow them to instantly see in the dark.

Fabulous fact, courtesy of the Wall Street Journal and Mythbusters, from the Children's Museum, Indianapolis.