Facebook's sprawling campus in Palo Alto, California was previously owned by Sun Microsystems.

Sun Microsystems was acquired by Oracle in 2009.

When Facebook moved into the office, Mark Zuckerberg didn't replace Sun Microsystem's sign. Instead, he flipped it over and put Facebook's name on the front.

Why?

The Sun logo reminds employees to stay motivated. It demonstrates what can happen when you're on top but fail to innovate.

Lev Grossman interviewed Mark Zuckerberg for Time. Grossman writes about the sign's symbolism here:

Because of the limits of space and time, a lot of Silicon Valley companies don’t build new headquarters; they just take over the discarded offices of older firms, like hermit crabs. Facebook’s headquarters used to belong to Sun Microsystems, a onetime power-house of innovation that collapsed and was acquired by Oracle in 2009. When Facebook moved in, Zuckerberg made over the whole place, but he didn’t change the sign out front, he just turned it around and put Facebook on the other side. The old sign remains as a reminder of what happens when you take your eye off the ball.

Here's the back of the sign: