I considered the sidewalk chalk idea, but thought the liquid chalk would be easier to get since there are a couple sporting goods stores near my office (I'm not quite sure where to get sidewalk chalk). I couldn't find any liquid chalk though, so will probably have to sneak in real chalk today and hope I can get away with it. I assume that sidewalk chalk is somewhere between blackboard chalk and "lifting" chalk in hardness, since I find it hard to imagine using blackboard chalk.
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