Around 2003, while I was a graduate student at Berkeley, my flatmate Patxi stuck his head in my door and said

Dave, what's a manifold?

So, I told him a bit about differentiable manifolds and their many uses. The next day, he said

I told Ushnish that I asked you what a manifold was. And Ushnish laughed, and said "I'll bet he told you, didn't he!" You know that I thought you were going to say something about cars, right?

And a day or two after that, I got this picture of Oolong the rabbit from Patxi. I put it on my door. It came down when I moved, but it has made periodic reappearances on my office doors ever since.