Air pollution isn't the same everywhere — the amount of particulate matter drifting through the air shifts depending on location and even the day of the week. To measure this, you coat microscope slides with petroleum jelly and place them in different spots: some indoors, some outdoors on window ledges or in open fields. After leaving all slides exposed for the same amount of time, you compare them against control slides kept sealed in a drawer. Under a magnifying glass or microscope, you count the particles stuck to each one. The results show which locations collect the most airborne dust, smoke, and soot — and whether weekdays differ from weekends.