A couple of weeks ago, Ed bought a whiteboard for our apartment ($8 at Walmart). He already had a tripod-style easel that his mom gave him, so this easel-mounted whiteboard now lives in his room. The idea was to make it easier for us to show each other things, and also as a general math-working space for him (I think).
Last night, quite late, he was eating dinner and I wanted to show him the smallest finite projective plane (the Fano Plane). It's really hard to show someone something on paper, because you just can't write on paper while holding it such that the other person can see it, at least not easily. This is doubly true if the person is trying to eat a plate of pasta at the same time.
"Want to use the whiteboard?" he asked.
It was our first time to try it together. He got to chill out sitting on his bed with the pasta while I drew and explained the plane (shown at left). And it was awesome.
I wasn't that into having a whiteboard, but now I can't see how we could live without it.