• Blame the thin, delicate layer of skin cells covering the lips for our human proneness to parched puckers.

Skin serves as a barrier between the body and the outside world.

Like an imposing bevy of bouncers at a concert, skin cells work to block all kinds of unwanted line jumping and exit hopping.

Skin cells have an obvious duty to keep outsiders from penetrating the venue they guard.

A more subtle but equally important task these cells perform is to keep valued guests, such as water, from leaving the party too soon and literally vanishing into thin air.

Lips, though, happen to be covered by some of the thinnest skin on the body, and the skin cells there simply aren't very good at this last task.

To picture a layer of skin cells, imagine a pile of snippets of paper that have been heavily colored on both sides with a waxy crayon, then pressed together into a thin sheet. The crayon coating would work both as a partial barrier against liquids and as a kind of glue.

No crayons color the body, of course, but surface skin cells coat themselves in oils and fats that serve a similar purpose by blocking the movement on water between cells.

These skin cell oils — just like crayon wax and (not coincidentally) the ingredients in lip balm — have a shared property. All are hydrophobic.

The word hydrophobic literally means "scared of water," but that's a trifle misleading. Lipids don't cower when rained on.

Rather, water molecules don't tend to mix with lipids for much the same reason that when two similar people hit it off at a party, they often neglect to mingle with an other guests for the remainder of the night. Other partygoers may still seem mildly interesting, but not enough to warrant working the room. Oil and water really don't mix.

When skin cells on the lips are in top condition, the oil coating keeps the cells in a linked, continuous layer, and water molecules continue happily mingling beneath the skin's surface.

Inside, ideally, the lips would be moist and comfortable; outside, the surface of the lips would feel perfectly smooth.

But remove that oil barrier — as often happens in dry weather when that top layer of oils gets thoroughly licked away — and the flat, scaly skin cells separate and flake away.

Water escapes from the tissue beneath and painful chapping is the result.

We reach for lip balm to soothe chapped lips because lip balms temporarily restore a lipid coating.

Beeswax and petroleum-based lip balms and salves work to seal water inside the lips. This helps for as long as the balm stays put.

This artificial coating — the waxier and greasier the better — briefly halts chapping and cracking and gives the skin a chance to heal.