Transcript:

Okay, here's a practical problem. You have a whiteboard on which you've written some stuff, maybe months ago, and now you want to clean it. You take an eraser, and you... it's not coming off. What do you do?

Here's a trick. Take a marker, a dry-erase marker, and highlight the stuff you want to erase. For example, I want to get rid of this 2, so I just go over this 2 with a marker. And then you take an eraser and you... magic.

The reason it works is the dry-erase marker has a solvent in it, and when you overwrite the stuff you want to erase, that solvent goes in and dissolves the older ink that has dried on the whiteboard. And when you overwrite that with the new marker and erase it, the solvent takes off the old ink also, and the whiteboard is clean.