With a permanent marker.
All over the freshly laid hardwood floor.
I am not given to hysterics as a rule, but I had them just then, I can tell you. Mercifully I discovered that rubbing alcohol plus a great deal of patience would remove most of the marks; then I discovered that nail polish remover did an even better job. So there I was, sweating with fever and feeling like I'd been run over by a combine, scrubbing big black scribbles off the kitchen floor with this nasty-smelling stuff and praying that it wouldn't completely destroy the finish.
A couple of hours later, I discovered that Nicholas had also decorated his light grey bedroom carpet with a bright yellow crayon. And that, it seems, is not going to come out.
Not a good day.