Why the first jack-o'-lanterns were turnips.

Go look at a photograph of a traditional Irish turnip lantern sometime — in daylight, with company. Where the pumpkin grins, the turnip grimaces. Carved from a dense, waxy root about the size of a fist, the old lanterns have pinched eyes, crooked teeth and the general expression of something that followed you home. They were not decorations. They were wards.

The custom comes out of Ireland and the Scottish Highlands, where the end of October marked Samhain — the old festival closing the harvest and opening the dark half of the year. On that threshold night, the boundary between worlds was said to thin, and things might wander. A hollowed root with an ember inside, set on a windowsill or carried along a dark lane, made a portable, frightening little light: part lantern, part scarecrow for spirits.

The name arrives with a story. 'Jack of the lantern' — Stingy Jack, in the best-known telling — was a drunkard and a cheat who tricked the Devil once too often. Barred from heaven for his sins and from hell out of the Devil's spite, Jack was turned out into the night with only a burning coal, which he carried inside a carved turnip, doomed to wander until judgment. Any strange light bobbing over the bogs at night was Jack — and a carved lantern on your own sill told him, and anything walking with him, to keep moving.

The pumpkin is the American remix. When Irish and Scottish emigrants crossed the Atlantic in the nineteenth century, they found a native squash that was everything the turnip wasn't: enormous, hollow, soft-walled, practically begging for a knife. The ward relaxed into a welcome. The grimace learned to grin. By the time trick-or-treating spread in the twentieth century, the jack-o'-lantern had switched sides entirely — from keeping strangers off the doorstep to inviting them up it.

Something of the turnip survives, though. Carve anything with a face and light it from inside, and for one flickering second on a dark porch it still looks like it might be watching the lane for Jack.