I got the following from wikipedia, which, as far as I know is consistent with Post-Crisis continuity and also remember reading in "Who's Who."
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The most common explanations for his invulnerability are Superman having a super-dense molecular structure and/or a supercharged bioelectric "aura" which acts as an invisible "force field" around his body within a few millimeters from his skin, and presumably within his body as well. The proximity of this field to his skin means that loose clothes, for example, may be burned off of him, while cloth that is close to his body is protected by the "aura;" This phenomenon is sometimes cited as Superman's reason for wearing a skin-tight bodysuit.
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When pulled away from his skin, the suit should have been easy to cut off.