To observe some of your own cheek cells, take a swab of the inside of your cheek and wipe it on a glass slide. Carefully place a glass cover slip over the cell sample, lowering it slowly from an angle so that you don’t trap any bubbles. Put the slide on the microscope stage and clip it in place. Use the shortest lens (lowest magnification) first and adjust the coarse focus knob until you can make out the individual cells. Switch to a higher-powered lens and adjust the fine focus knob so that you can see the cells in greater detail.