The gas has been turned off for the night, the old wooden shed laboratory is dark and one small lamp still burns on the workbench. Pixel Marie Curie has a full tray of sample cards to sort before morning: flasks, crystals, notebooks and vials, each with exactly one twin, all lying face down in rows. Students walking past at night have a name for that one glowing window: the Lamplight Shed.
Only the patch of bench under the lamp can be seen, so she moves it along the rows card by card, and the last card she turned stays softly lit. Every pair found is a step closer to a tidy lab, and each finished tray brings a bigger one. When the clock runs out the lamp oil is gone and the night is over. How many trays can pixel Marie sort in the dark?










