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:

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

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

Update 2025: They still sell stickers, magnets and T-shirts featuring the warning! The T-shirts are printed with the words, but not the cows, in glow-in-the-dark ink, so at night, the cows are in fact invisible. I’m tempted…

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