“Right,” Murphy said. “You get them from the teacher when you get an A.”

“Another example of symbols having disparate meanings,” I said. “But look here, in the middle.” I filled in the closed shape in the center of the star. “That’s a pentagon shape, see? The center of the pentagram. That’s where you contain whatever it is you’re trying to contain.”

“What do you mean, contain?”

“A pentagram like this one is a symbol of power,” I said. “It’s got a lot of uses, depending on how you employ it. But most often you use it to isolate or contain an entity.”

“You mean like summoning a demon,” Murphy said.

“Sure,” I said. “But you can use it to trap other things too, if you do it right. Remember the circle of power at Harley MacFinn’s place? Five candles formed the pentagram on that one.”

Murphy shuddered. “I remember. But it wasn’t this big.”

“No,” I admitted. “And the bigger you make it, the more juice it takes to keep it going. I’ve never, ever heard of one that would take this much energy to activate.”

I drew little X shapes at the points of the star and drew the chalk from one to the next, thickening the lines of the example pentagram. “Get it? The beam streamed from one reflector to the next, melting holes through the building as it went. The reflectors formed the beam into one huge pentagram at ground level, more or less.”

Murphy frowned and squinted at the simple diagram. “The center of that shape couldn’t have covered the whole building.”

“No,” I said. “I’d need a good map to be sure, but I think the center of the pentagram must have been about twenty feet back from the front door. Which is why only the front half of the building collapsed.”

“The explosion came from inside this pentagon thing? Magical TNT?”