Up at the visitor’s center for the Mauna Kea observatories, there’s a sign that says, “Beware of Invisible Cows.” It was dark when we were there, and I tried to get this picture without using the flash since there were people with portable telescopes ten feet away, so it’s really blurry:

The actual invisible cows sign (blurry)

Fortunately someone in charge recognized the humor value, and the visitor’s center sells bumper stickers:

Beware of Invisible Cows
Why invisible cows?  It's dark and foggy.

Of course, it turns out other people, visiting during the day, have snapped better pictures of the sign.

Note: Our visit to Mauna Kea was on Saturday, April 9, 2005.