To best create the bubble for the seal, it is a subtle two-part process. First blow quickly (not hard) to let a thin, high-topped bubble. Then stop blowing just long enough to let the very thin spot become hard. Finally, blow a long steady breath that expands the thicker (and therefore still soft) part of the same bubble. What this does is to expand the width, of the bubble and maintain a very thin top surface membrane. You can control the width of the bubble fairly accurately in this manner.
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