The tip of a properly inserted stopper will create a bull’s-eye pattern at the bottom of the well. To view this bull’s-eye pattern, turn the plate over after inserting the stoppers and tilt the plate at an angle. You will be able to see this bull’s-eye pattern in the center of each well through the clear bottom surface. Stoppers that have not sealed well can be re-inserted until the tip is properly placed. See a video of the technique here.
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